1) If you can jiggle, wiggle and move the drive shaft 1/4" where it attaches to the flex disk something is wrong. It should be tight with no play. Visually inspect the flex disk and look for any cracks or chunks of rubber missing from it. If externally it looks okay and you have play, the bolts may be worn where they go through the the bores of the flex disk. Read my post in today's thread on the subject of replacing the flex disk on a 300E.
3) I don't know about your mounts. I've had rear springs squeak and a little WD-40 corrected the noise.
4) Mixing tire brands front & rear will in most cases give you skittish handling. For street use I'd never mix tire models within the same manufacturer let alone mix brands. If you can swap four wheels from one of your other cars or borrow four wheels from a fellow benz0phile this is easy to check out.
5) Steering free play is not related to rear suspension problems. It could be one item in the front end or just the combined wear in all the components that make up the steering assembly.
6) I don't remember how my 190E shifter is bushed. But all my other cars have a nylon bushing on the end connecting to the tranny. If there is play at the connection, it would seem likely that a bushing may go there. Did you get a picture parts book with your car? If so check and see what and where the bushings are.
7) I'm not an AC pro, but sweating oil would indicate a leak somewhere.
Sorry I can't be of more help.