I am originally from Hawaii and spent four years in Miami, FL before moving to San Diego.

I would start by looking at ducted mini splits, heat pump water heaters and induction cooktops.

This was achieved by converting a weird storage room into a "three seasons room", we added additional value by putting in a heat run, making it a 4 seasons room and adding that sqft to the house.

Any suggestions as to 1.) how to go about installing more vents or getting more heat/AC (I don’t want window units) 2.) what do do about the mystery intake vent/hole in the floor?

Every decade has it's own points of concern including heat, electrical, plumbing, insulation and foundation.

AC units should have return air also, not just the supply ducts like many heat only systems. 

Thanks All, @Marcus Auerbach I was too heated at the time.

Tenant is responsible for making sure filters in the heating and AC unit are changed regularly; and to make sure heat is not below 60F during the winter.Here's the listing: https://www.realtor.com/reales... 

Not sure what kind of climate the property is in, but paying for part or all of your tenants' utilities can get expensive, since they have no reason not to (for example) run the heat full-blast with the windows open in January. 

Typically there are laws about the property being maintained in a habitable condition with a limited time frame required to complete habitable related repairs, i.e. electrical, plumbing, heating, cooling, ...As a property manager, I agree with you that there seems to be "a lack of quality assurance" as well as a lack of communication and sense of responsibility from your PM.