The main problem I see is that it seems like you're making a lot of assumptions based on geology here on Earth, such as which minerals are likely to be present at sites with particular geologies. Doesn't that depend a lot on the early planetary formation?
Chemistry works the same everywhere. What elements readily form compounds with other elements is the same everywhere. At what temperatures minerals begin to crystalize out of magma is the same everywhere. Etc. Economically valuable deposits of resources are locations in which chemistry tended to concentrate that mineral and leave it at an accessible location. The same parameters must apply to the moon just like on earth.
Also, correct me if I'm wrong, but I thought I had read that, just like you say with the Moon and heavy elements sinking to the core, the exact same thing happened to the Earth, and as a result, we have no heavy metals, including iron(!), accessible here on the crust left over from the formation of this planet.
90% of the mass of the Earth is oxygen, iron, silicon, and magnesium. And these chemicals tend to form compounds with each other. Consequently it's impossible for "all of the Earth's iron", for example, to have sunk to the core. More to the point, these oxides aren't as dense as the pure metals. For example, in the crust a lot of iron is found as limonite (that yellowish-orange color you often see in clays), which can be nearly as light as quartz. The largest single mineral component of the mantle (and thus the Earth) is olivine (commonly known as peridot when sold as gemstones), a magnesium iron silicon oxide.
Unlike the outer layers, earth's core is predominantly metallic iron, not oxides, and thus far denser. It's also highly enriched in many heavier elements which either don't readily oxidize or form heavy oxides. For example, platinium is found at about 5ppb concentrations in the crust, but is believed to be about 6ppm in the inner core, over a thousand times greater concentration. Uranium, thorium, gold, and countless other elements are vastly more common in the core than the crust. That doesn't mean that they're absent elsewhere. Even ignoring deposits from bombardment, you will often find small amounts of rarer elements in minerals with elements that they're chemically similar to.
You can see the nature of mixtures in what erupts to the surface as lava - an igneous flow will ultimately crystalize out into a wide range of tiny mineral grains - various feldspars, quartz, various iron oxides, etc. These crystals have different densities, and they're made from elements with different densities - but the forces keeping them in solution are greater than the forces working to fractionalize them. Differentiation inside magma takes a long time - for example, to get basalt rich in large olivine crystals, like picrite, the magma has to sit and slowly cool over many thousands of years, allowing the olivine time to crystalize out and the crystals time to settle to the bottom without the bulk of the magma hardening and trapping it - then the upper olivine-poor magma erupting, then the olivine-rich magma erupting (again, all without hardening to the point of becoming trapped in the magma chamber).
Or, to put it another way: salt is heavier than water, but the bottom of the oceans is only slowly increases with depth (and is highest near the surface where water evaporates, but that's a side point). It's a lower energy state for the salt to dilute than to all collect at the bottom.
nd that all our valuable ores (iron, gold, silver, even tin and lead) came from asteroid impacts over the eons, which is why they're concentrated in particular places.
That's not why elements are concentrated, as a general rule (although there are exceptions). Most concentrates are due to various geological processes involved in preferentially enriching or depleting minerals from a bulk. For example, you know the old miners' saying, "Gold wears an iron hat" (gossan) - do you know why that is? Iron and sulfur-rich rock tends to contain pyrite. Pyrite plus groundwater (and with the aid of bacteria) over time produces sulfuric acid. The acid leeches the rock around it. The minerals that dissolve in the acid concentrate where the water reaches the surface, often leaving iron stains/deposits, along with deposits of other dissolved minerals such as copper, which tend to precipitate out together. The minerals not eaten away by the acid (quartz and resistant minerals, including gold) tend to be concentrated underneath
(Totally unrelated side note: I actually have an iron bog on my land, and as a geology nut I find it fascinating, when most people would just find it disgusting muck ;) One of the IMHO most interesting characteristics is what looks like oil slicks on the surface. But if you actually touch them, you see that they're not a liquid, they're an iridescent film. It's a consequence of iron-metabolizing bacteria oxidizing Fe+2 to Fe+3 and releasing goethite as a byproduct. :) )
That's just one example of a process that concentrates minerals - there are countless. But in the world, wherever something is economically exploitable, there almost always was some sort of geologic process that highly concentrated it there.
And while it sounds like we understand a good deal about geological processes, I'm not so sure it's that complete: didn't we only figure out the southern part of Mexico was formed by a giant asteroid impact within the last few decades, and that that was the cause of all the caves and such along what's left of the rim of the crater?
The Chicxulub crater was discovered four decades ago, in the late 1970s. And in terms of our understanding of geology in general, that's a huge length of time. Remember, it wasn't until the 1960s that plate tectonics and the concept that large bodies still impact the Earth became the scientific mainstream. Modern geology is somewhat "young" compared to the other sciences. The foundations of astronomy became solidified people like Galileo and Copernicus, classical physics with Newton, etc, but even around 1800 variants of a "Noah's flood" theory were still mainstream in geology (the "Deluvian" or "Neptunist" theory), and the fact that fossils tended to align into layers was a newly discovered curiosity (commonly explained by the Neptunists as due to how they'd settle out in the ocean or flood).
Anyway, with asteroid impacts, the moon is full of them, as we can see easily with a small telescope, and unlike the Earth, there's been little tectonic activity and no atmospheric or water-based erosion. So wouldn't that mean that each impact site could potentially have a lot of valuable ores?
Impacts really don't work that way. Impacts over a certain size are basically converted into plasma on impact and explode. Even on earth, this does little to create economic deposits of minerals, as it's so spread out. On the moon, with low gravity and no atmosphere to get in the way, you're spreading the body of your impactor over vast distances.
There is one way in which impactors do sometimes produce valuable deposits, mind you. The aforementioned Sudbury deposit is a good example. The impact, as mentioned, blasted material all over the Earth - it was utterly obliterated. But the impact was powerful enough to create a melt pool all the way down to the upper mantle. The process in which it differentiated and cooled is complicated, but you can read about it here. But the key takeaway is, the deposits are overwhelmingly from the crust and mantle, not the impactor.
You also mentioned titanium being plentiful there. Wouldn't that be a good enough reason? Titanium isn't exactly cheap here.
No. First off, even titanium metal is cheap here - about $10/kg, which is nothing (platinum, by contrast, is $30000/kg, and there are countless things far more expensive than platinum). When your launches cost tens of thousands of dollars per kilogram (and even after future process refinements will still probably cost thousands), mining something worth $10-20/kg obviously is not going to pay off. But more importantly, most of the cost of titanium metal is refining titanium dioxide. Titanium dioxide, after milling, costs about $5/kg. It's so cheap that it's the predominant white pigment used on Earth (white paint, sunscreen, etc... pretty much anything that you want to be bright white uses titanium dioxide).
How much fuel are we wasting because we still build cars out of steel instead of titanium?
Honestly, the main reason cars are mainly built out of steel is process-based. We have long-established historic processes for mass-manufacturing vehicles out of steel. Composites are generally much stronger per unit mass, and the raw materials costs can be kept lower than steel (glass or basalt fiber rather than carbon, vinyl ester instead of epoxy). But it's much harder to mass produce out of composites than steel - it's hard to get automated processes to produce parts of consistent quality.
Titanium is $10/kg. Aluminum is $1,50/kg. Steel is about $0,35/kg. So while none of these are "expensive" materials per kilogram, obviously when you're mass-producing multi-tonne objects the difference matters. Automakers are more and more incorporating aluminum into vehicles to (borrowing from Lotus) "add lightness". But it's also important to know that these materials aren't just simple substitutes for each other. Aluminum is 1/3 the mass of steel, but also not nearly as strong, and with a much lower melting point. Titanium has a high melting point and is roughly as strong as steel. Aluminum is generally harder to weld than steel, and titanium much harder to work with in general than steel. Also there's the issue of experience - there's more people with experience working with steel than aluminum, and vastly more with experience with aluminum than with titanium. The rarity of people with experience working with the metal makes such employees more expensive to employ.
The (former) wide availability of titanium in the former Soviet Union was not due to the widespread sourcing of titanium ore, but rather the communist government prioritizing it as a war resource and pumping large amounts of money into its production.
Venus is completely inhospitable at the surface for both humans and machinery, so I have no idea how we'd exploit mineral resources there.
I already discussed this. Google "phase change balloon". Keeping things alive for hours at the surface is not a problem - the Soviets did this with the Venera landers. You simply have to have a thermal mass to act as a cooling reservoir. Once your thermal reservoir becomes too hot, you have to return to altitude to cool or replace it before making your next "dive". It can be thought of as rather akin to mining the seafloor. Due to the limited time per dive, this would be grossly impractical to try to control such equipment from Earth - you need people "locally" to get rid of the latency issue. Hence the reason why there's actually some logic to putting humans on Venus.
If the Soviets could run sampling equipment on the Venus surface on a shoestring budget with 1960s/70s technology, there's no reason that we couldn't have surface mining equipment today. But obviously, every case comes down to economics.