General consensus is that class changes are possible if done before takeoff with consent of ISM, but those are then permanent for the flight. i.e. you can't swap halfway.

It makes perfect sense. Otherwise many people would do this and swap halfway so they can each get a few hours of "flat bed" sleep.

What the OP is asking for is not reasonable and according to most people's experience (I have never tried it myself) is not allowed.

Seated together during descent......maybe. But absent a compelling reason I don't see why they would let a Y pax sit in F even for descent.