If I'm theoretically one move from a target that requires some amount of power to land but not a lander burn, this means I can just "say" which thruster I'm going to use and land without wasting fuel?
Yes. Incidentally, this is a major difference from 2nd edition: you cannot make fueled liftoff/landings anymore. Either your thrust is good enough or you can use factory assist or you are stuck.
What if I need afterburner to reach said power, do I then spend the fuel for the afterburner but no more?
Can I jettison my engine and do an aerobrake landing with no power at all?
Yes, if there are no burns along the aerobrake path.