How can I relocate a receptacle that cannot be removed from the junction box?
I would suggest trying harder to remove the screws before I started to rip up the floor; ripping up the floor to fix an electrical outlet/box just seems way to extreme.

If the screw head is stripped, use a specialty bit to remove stripped screws.  If it's rusted, try to use some penetrating oil (turn off the power first!).

As a last resort against the screws, you can drill out the screws and use a new tapping bit to create new larger threads for new screws.

If all that fails, just bust up the outlet (again, power off!) so that you can remove it or at least get access to other parts of the box so you can cut/grind whatever you need to remove it.