Easy way to remove the screw on the right side holding the heater? No.
Long screw driver, pushed down firmly to avoid stripping the screw will
do it, better is small 90 degree ratchet screw driver setup.

With cover off and those two screws out, disconnect hoses and plug, very
little coolant should escape.  Whole heater comes out on Westy setup.  I
understand that there are other problems associated with standard
Vanagon setup and that requires some disassembly of the seat.

No differences between an '85 or '90:  have worked on both, in fact just
permanently removed the rear heater from my '90 Westy about 10 days ago.