Is it possible you disturbed a wiring harness or connector while doing the other work? 3 experiences I've had might be informative:

on one car the dealer had replaced the trans before I bought the car used. They nicked a wiring harness under the hood and damaged the engine temp sensor with the result that the dashboard temp gauge worked erratically. Easy to fix once I found it (not a Volvo).

poorly seated large under-dash red connector after replacing evaporator. The trans sometimes did what yours does (this is my 850).

stuff spilled in center console on a 240. The auto trans gear position sensor is there on that car and the starter sometimes wouldn't engage (as if not in park or neutral). On the 850 does your mode switch and all window and mirror switches work? Also possibly a bad ground to the console especially if any work was done there in the past.