Elecric seat overide

IF the seat is functional, but doesn't operate unless the key is in acc/on, then a fairly simple procedure would be to install a momentary push button/toggle switch that powers the seat. It would then be usable at any time..

I would find a switch that has both NO (normally open) and NC (normally closed) connections where the NO is also MOM (momentary). This would set it up so the existing wiring is disconnected while in "override" mode thus would prevent back-feeding of those other items that are also controlled by the acc/on position.

If the the seat is functional...

They all use a screw drive that is driven by the motors
With out disconnecting the drive there is no way for the sear to slide

You did not state what your problem is, if one Motor is dead then it will have to be replaced or repaired,
If you have lost power to the seat and can't find the fuse, you could cut the wiring and use jumper cables to power the seat,
But rather inconvenient if you want to move it often

I don't think so
I have never seen an electric seat with an override
it would have to disengage the motor,

the model/brand off the seat and 'goggle it'