I've done a bit of this.
For every adjustment you will find there is a reaction someplace else.
Raising one side may also change the tilt of the room.
Lowering one side may square it up but may also cause it to exert too much down force on the floor as it comes in.
It ends up being a trade off between being square and true and operating without negative effects.
Make small changes. Dont try to level the slide in one adjustment. Make a small adjustment and then run it in and out a couple of times while watching for binding or clearance issues. Keep making small adjustments until it's as good as it can get. Dont expect perfection, you may not be able to achieve it.
Does the slide sit too hard against the carpet on one side when it in or leave an indentation? If so I would raise that end first.
As for the squeal, some slides have hard plastic guides that rub against the upside down channel on each side of the slide. It's just plastic against metal and so it makes that awful noise. You can lube it but it wont last long and the noise is back.