Problems with 80' 300sd

I have returned to ask a couple of questions about problems I am having.

A) The first thing is that the engine does not want to cut off unless the climate control is set to "BILEVEL". Which I have heard was a vaccum realted function. But I cannot find a leak.
B) Also when the air compressor is engaged, it makes this horrible rattle; it dissappears when the engine is idled @ 1000 rpm. The AC still blows cool air.
C) My fuel return lines are leaking, and I am about to go buy a replacement, could this have some effect on shutting down.
D) The vehicle shakes really really bad unless kept at about a steady 900-1000 rpms.

I am sorry this is so long, but it's hard to find a mechanic who is willing to work on my car. So hopefully, I can find some help here.
Thanks,

Ty