Would anyone know how to fix any of these automotive problems?

I’ve got my heart set on a 1990 Pontiac Grand Prix Turbo, and since GM was never perfect with reliability, there are a few niggles in the package. None of these are seemingly detrimental to the mechanical well-being of the car. If you know how to fix any of the following, drop a line:

1. The up & down switch for the automatic side mirrors will only work after each mirror is moved left, then right, and then either the up or down button is pressed repeatedly and quickly.
2. The low coolant warning light comes on almost every time the car starts, though the coolant level is fine. (I haven’t checked it personally because I can’t see anything on the reservoir that indicates where the fluid should be, but current owner says that he has checked the coolant reservoir whenever this has happened to find that there was plenty of 50/50)
3. The gas gauge warning light is always on.
4. The Check Gauges light is always on, and this is probably caused by #3.
5. The driver’s side power window motor is incapable of moving more than an inch without human support.
6. The RESET button on the informational console is broken, meaning that the service reminders on the console cannot be reset. That was kind of a given.
7. According to the current owner, if the battery is disconnected and there is no charging-battery-thing plugged into the DC socket, something to do with the computer, timing, etc. will be reset and do not correct themselves for months. Short answer; get a charging-battery-thing, but I would like to know if this can be fixed.
8. The physical speedometer is 5–10MPH optimistic, though the electronic speedometer in the HUD is correct.
9. The seatbelts usually feel slackened at the shoulder. I’m skinny and small, so the fix is probably to eat at McDonald’s for a year or three.
10. The lower front bumper (the very bottom piece which a lot of TGP owners nick by hitting a sharply-sloped entryway) seems to be separated from the main part of the front bumper at the far corners.
11. The rear bumper sags where it meets the trunk lid at the right side corner.
12. The clear-coat on the top of the rear bumper has been partially consumed by the sun.
13. The clear-coat on top of the side mirrors is also corroded.