How to settle A/C and heat wars with roommate?

Your roommates are gonna have a rude awakening when they start living in the real world where tenants pay their own utilities. Keeping the heat at 75 is not normal or reasonable. Most people keep their heat at 68-72 in the winter. They can always wear a sweatshirt, but you have no more layers to remove. You could always walk around butt naked. If they don’t like it, tell them to turn the heat off. No sane person keeps the heat on when it’s 80 degrees outside. They can just open the windows. At least that way you’ll get some air circulation. You can always get a window AC unit for your room. It’s unfortunate that the heat vents don’t close, maybe you can cover yours with something. 

It’s always gonna be hotter upstairs. Because heat rises, if you open your window on the 2nd floor, the hot air will naturally flow out the window, and the air from downstairs will flow upstairs to replace it. That will cause fresh air from outside to flow in downstairs through any gaps and cracks (or open windows). Instead of cooling off your room, it will cool the downstairs, which will only encourage them to increase the thermostat. 

I think your best solution is to make a compromise that the thermostat remain at 72. If they can’t abide by that, then every time you feel warm, turn it to 65. A normal temp of 72 is comfortable for most people. If they’re cold, they can put more clothes on. There’s a reason most people wear sweats when they’re cold. They should get used to it because in most apartments, if the heat is included, the tenant does not control the thermostat. And I can promise that no landlord will keep the entire building’s heat at 75. If they pay their own heat, they’ll get a big shock when they owe hundreds of dollars for one month. 

My daughter and her dorm roommate were a bit like this. My daughter was always up and out early, so as she left she turn the thermostat back to her roommate’s liking. When roomie got up the room was at her preferred temp. When my daughter got back to the room, roommate was  at her classes and my daughter would turn the thermostat back to where she liked it. No clue what they did over night when they were both there. Do you have any RA that can mediate for all of you? Call housing and ask. 

You can buy a portable air conditioner for under $100.  I suggest you invest in one.

maybe you should live with other roommates

You might want to point out to them that heat rises. If they're "cold" at 75° maybe they should simply wear more clothes or perhaps use small space heaters? Your only hope is to keep doors open and hope that helps the heat circulate - close your door and you suffocate.