Insulate portable AC vent duct

Solar load (which heats the building to 95 on a 68 degree day) is a real problem. Insulation will make things better, not worse.

The #1 thing you can do is paint the roof white. Yes, I know that's hard and weird, but it really does work.

Soffit and roof vents, obviously, make it a lot harder to heat a shed. Maybe the architect was concerned with condensation inside your shed, and want to make sure moist warm air from the daytime is cycled out by nightfall.

If you're going to hang a ceiling and have a void between ceiling and roof, it helps get rid of solar heat if you move a lot of air through there. That will carry away the heat that comes through the roof before it reaches the ceiling. Of course this would be unhelpful if you are trying to heat below the ceiling.

If the apartment is a 8" thick "cinder-block" wall with no insulation, then it's R value is likely around 1.5-2. If it's a concrete wall, it's R-value is around 3.8-4.

In any case, 1" thick polystyrene foam has an R-value of 5, and ½" drywall is about .5, so adding the foam and drywall (combined R-value of 5.5) over your existing drywall will more than double the wall's insulation value (in the case of concrete) and almost quadruple it (in the case of cinder block). All assuming there really is no existing insulation.

I would say the wall will become dramatically cooler.