I’m not an HVAC technician so I wouldn’t know exactly, but I think there’s two things to consider: static pressure and duty cycle. Static pressure is the fan working hard to spin which causes wear on bearings. Duty cycle is how long the machine runs. If your duty cycle is too low then the machine is turning on and off quickly which is bad.

You could look at the size of the unit and there’s some rules of thumb that translate size, climate, and square footage of just the areas you leave the vent open and there are online calculators you can use.

I think the only way to know for sure is to measure the pressure drop across the intake and outflow and see what the fan motor is rated for.

Depends on the unit and whether it was over sized or under sized for the space, but restricting 60℅ air flow is a lot.

Another thing to consider is that if you later want to heat up the entire house, the unit has to work harder. Sometimes it can be more efficient to just keep the house close to your target temp.