Wow. I'll try.
The 92 300E should Idle above the 600 mark. The engines are always adapting to wear so they can have some variation in idle caused by wear and the temperature of the outside air.
Sometimes you can have a faulty idle speed acuator or air plate sesor or throttle switch. (Needs to be diagnosed by a tech) I only do Mercedes so I don't know about Qestion 2.1.
Anti slip differential.
Very complicated if not trained how to repair. Go to tech for repair.

Doubt it
It is a good idea to replace the fuses on the older Mercedes, they do deteriorate with age.

The battery may be old. Have a capacity test done on it.

------------------
Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM