your trunk is vaccuum operated. But then you have a more fundamental problem - you can't unlock it with your key.

something must have jammed into the lock or you have been trying it the wrong way. The key cylinder should be vertical at all times unless you double-locked it which then it's horizontal. But either way you should be turning counter clockwise until it unlocks. Once it unlocks it does not pop open as if you pressed the trunk release in the car, you have to unlock it and press the whole key cylinder inwards with your thumb (key is not necessary after unlocking)

so I'm starting from the dumbest reason first, did you double lock the trunk by mistake before?

If yes, turn the key 1/3 of a turn to the counterclockwise direction then push the whole black circle with the key cylinder inwards into the trunk and it should pop open.

Open it up, THEN look for vacuum/solenoid problems in the automatic trunk release mechanism.

Can you fold the rear seat without opening the trunk? I would think not possible?