The room is on the north east side of the house with several windows and a walkout door.
I think that this contributes to the temp differences in this area.
your quotes are all over the place.
qoute 6 "We pulled the grill and he felt the joist areas weren't sealed very well and may be pulling unconditioned air from garage space etc."
this would be a concern. if you are pulling low temp air into the return..then it will have an effect
on comfort of the house. I'd check into this with whichever bid/company you decide upon.
at 5 tons for 3200 sq ft..I'd think that someone sized the system.
usually on 3200 sq ft you see multiple systems. and 500 sq ft per ton sizing.
could be problems with ducts & returns. did any of them check out ducts for leaks or restrictions?
best of luck
The cure of the part should not be attempted without the cure of the whole. ~Plato