Agh, wtf is a salad? Come on posters, use links so those of us without a clue can figure out what's going on!
Not to mention that the first sentence on the linked article describes it concisely.
Local overheating, hard radiation hits; voltages can actually be pretty high depending on string length and your orbit. Spacecraft experience high differential charging depending on the plasma environment through which they are passing. Outright cell failure is relatively rare though; typical failure mode is degradation below the operable voltage.
I don't know the underlying physics, but radiation (including solar radiation), especially at high temperature, causes the junctions in the solar cells to become less efficient over time. Less efficient cells generate more heat, which increases the rate of deterioration. Eventually the open-circuit voltage of the cell drops so low that it is below your spacecraft power bus and you stop being able to pull power off the array.
Another failure mode is when individual cells short-out, which happens when the junctions just straight up burn out. This generally results in the loss of individual cells, which lowers the voltage of the array or, depending on architecture, may take down that cell's entire string.
L1, L2, and L3 are weakly stable; think being at the top of a parabola. It doesn't take much effort to keep yourself there, but you do have to reject orbit perturbations. L4 and L5 on the other hand are actually stable, which is why trojans collect there. Note that there aren't any natural equivalents to trojans at L1, L2, and L3.
Sorry you said L1 and L2; ignore eclipse season comment.
Solar cells will degrade to the point they can't supply keep-alive power to the spacecraft; batteries will degrade to the point they can't sustain the spacecraft through eclipse season; electronics will accumulate more and more total ionizing dose, single event upsets and latchups will become more and more frequent, and things will basically stop working. I don't think anything we've launched will come within an order of magnitude of a millennium.
We don't design LEDs into our own boards, and we explicitly remove them from COTS boards that we use. Generally speaking the diffusers on LEDs outgas, meaning a) they are depositing materials on your spacecraft surfaces (bad) and b) could result in a shorting risk (also bad). There may be space-grade LEDs that big-space (think Hubble, JWST, Voyagers, etc.) use but I would be surprised. There's simply no need.
"Is it plugged in? Is it turned on? Is it on frequency?" solves about 99% of basic device connection issues. An LED will make one very short portion of that slightly shorter, and then only when testing on the bench, since you can't see it as soon as you box it up. As soon as you can talk to a device, you are able to run a long form functional test on it, exercising every part of the design and ensuring everything is working correctly. If it passes, you're good. If it fails, you pull the unit.
For ground support equipment, yeah sure, throw an LED on every rail and switch output.
They don't re-use those (yet), but they don't burn up on re-entry, they parachute-drop in the ocean. Every Crew Dragon flight gives them valuable engineering data and experience on their heat shields.
It's a radio... for talking to God!
What does God need with a radio?!
As far as I know, the only purpose of the barge is for technical development; the ultimate goal is return to launch site. I have no citation for this, but my intuition is that he won't try to deal with the Falcon Heavy boosters until the Falcon 9 and Falcon Heavy core are reliably returning to base and possibly even being reused.
The launch profile shouldn't change all that much for the Falcon Heavy; it's just sending more mass to the places Falcon 9 goes today.
It's not a secure password unless it is randomly generated. There are tricks you can use to make it more memorable, like using diceware instead of characters and numbers, but fundamentally if you came up with it, someone else can guess how you came up with it.
Eh.. launch vehicles are already an inverted-pendulum problem; I don't think the guidance and navigation would be an issue. I suspect the thrusters, even that size, simply don't have the necessary control authority. Besides, the ultimate goal is landing on land, so you're better off getting the rocket to be able to handle it itself. I'm sure it will be a lot easier to convince the powers that be that they can land on land safely when they can demonstrate doing it on a floating barge a few times.
That's the video from the previous attempt, which the OP was already referencing. There's no video from this attempt yet, just a couple pictures. Musk has already said we have to wait until the Just Read the Instructions returns to port for video.