1. Incorrect, they cannot, there are many levels of integrity. Have a look at the model for IE protected mode for instance. Normally you need the DEBUG privliege to read anywhere, this can only be granted by an elevated account.
2. Any installer will usually be prompted to elevate, through the mentioned heuristic. However you don't have to agree to this and instead install the APP into _non_ system directory. Very much like unix works, actually exactly the same way.
3. No it cannot, a non admin process (started by a non elevated admin, the normal with UAC enabled admin account) cannot send messages to an elevated process.
4. Nope it doesn't, it uses heuristics to figure out if the program has compatiblity issues and thus would need elevation to work. Vast difference, it will never elevate without letting you know and agree to it.
Windows 7 can be set to full prompt like in Vista, I would recommend this (which would close the mentioned attack, if the attack is possible in the first place, haven't tried yet). If you are hit by UAC prompts daily you are doing something wrong or something out of the ordinary. There are however exceptions as many games today still required admin to run, completely screwed up. They even have checks to make sure you elevate, some even go so far to recommand turning UAC off. These are the kinds of evils that should be fought, not good intentions like UAC in a difficult situation.
However you can easily setup shortcuts to start app's that need elevation so that it will not prompt you. However this setup _must_ be done when elevated. This will reduce the daily prompts, but it will still leave you more vulernable as the program/game you are running can now be attacked with much success.
Most people get UAC technically completely wrong. It is not like a sudo system, it is more clever than that and it needs to be, because MS built themselves into a corner by not getting it right from the start.
In an admin UAC enabled account your account is associated with 2 security tokens, one elevated and one "normal". All application you run and all activities you do is handled by the normal token. However whenever the UAC elevates you (through your consent) the higher priviliege token is used.
UAC can be changed to work exactly like a sudo system, however a sudo system cannot be set to work like UAC.
A sudo system is way more annoying than the UAC system, if MS would have configured this by default it would crippled even more users. Just like it would cripple people on a unix system (app-get requires root just like installers on windows gets elevated). MS should have been much stricter from the beginning so they wouldn't have ISV etc implementing apps completely in the wrong manner.
Any sys-admin worth his salt will not allow "normal" users to run under an admin account, not even in a XP environment. However with UAC and Vista they will at least gain the benefit of being able to easily elevate and install apps for the users, either physically or through AD push. Physically is useful when a user needs special software that the masses don't.
UAC can be set to ask for a password even for an admin, I wish this was default, but nothing prevents you from changing it. Or for admins to enforce it.
UAC can be set to auto elevate for the built in admin account (like logging in as root) but default it "secure".
UAC can be set to not use the secure desktop, thus allow script attacks. This is normally not the default mode, however in Windows 7 it can be default if the hardware you run on is listed as being too "annoying" when switching desktop. Due to inherently bad graphic card's in some low end laptops.