Ah Ha, been there - done that on both your issues.

1. I had the steering-wheel squeek, very hi-pitch just under the horn.
It was the carbon brushes that make the electrical connection to the air-bag.
There is a little copper disc on the steering shaft, and the wheel contains some little carbon brushes (similar to an alternator commutator).
As you turn the wheel, the brushes slide on the copper disc to make the contact. I took the steering wheel off (including the air-bag) cleaned up the disc and brushes, and the problem went away.
You can buy new contacts, but they are pretty $$ as I remember.
Pulling the wheel is straightforward, but be sure to disconnect the battery
and the the air bag actuation connectors under the right floorboard.
It's my understanding that the bag actuation circuit contains a capacitor which can fire the bag even though the battery is disconnected.
Read up on air bags in the Forum, don't want one going pop when you pull the wheel..

2. I hate that rear-end sag. I am always amazed when I see a late-model BMW with about 10 degrees of negative camber on the rear.
They must wear out the bushings in less than a year.
Two things will bring you back up:
a. Replace the big subframe bushings just in front of the rear wheels.
There are plenty of how-to's on this forum. Throw on a rear diff rubber support also.
b. If you need more adjustment (and you probably will) pull the springs, and insert thicker rubber pads on the top of the spring.
MB offers 3 thicknesses, and they can be obtained at the dealer.
do a search here on that also. The pads have markings on them that you can see if you crawl up there and look carefully, so you can discern which thickness you now have, and buy a thicker one. I think it's 3 dots, 2 dots, etc. which correspond to 9mm, 5mm, etc.

When completed, you should have virtually zero camber, and the height
of the rear fender well should virtually match the front.

Good luck.
DG