Heater: What's the problem you're trying to fix? The center vents could be a bad vacuum pod or it could be the temp controller thinking it should be in heat mode and therefore no air should go to the center vents (for MBs of that vintage, that's a feature).

Operating Temperature: Seems high to me unless your commute is uphill both ways. Make sure the clutch fan engages when it should. Make sure the electric fan engages when it should. It might be worth pulling the radiator for a good cleaning. Clean the condenser core while the radiator is out. Consider Redline Water Wetter and no more than 40% coolant. Less if it will work where you are.

Blowby: With the engine running, plug the hose from the valve cover to the air cleaner. Measure the time it takes for the engine to shut itself off. Report back.

Gas Milage: Try running it on Diesel Keeping the engine in a good state of tune is the best you can do. Check fuel filter condition, valve lash, timing belt stretch, IP timing, etc.

Rough Idle: Replacing the rack damper is easy. If the exposed end of your rack damper is silver in color, consider a newer style replacement with a gold color end. Check the condition of the engine mounts and engine shocks. Don't forget the transmission mount. If you don't know the history of the car, it might be a good idea to have the injectors pop tested and balanced for peace of mind.

Sixto
95 S420
87 300SDL