The issue is, that’s there’s no rule in the PL handbook that caters for a scenario like this, so your assumption is wrong, as the existing rules only cater for a completed season.
So the PL will have to collectively agree a solution should the league be unable to complete, same as its having to do re: the restart and neutral venues etc.
The FA can’t impose anything, it can merely threaten to not sanction a solution that doesn’t involve relegation. Neither of you have countered any of the points made as to why a PPG just doesn’t stack, notably the idea that 3 PL clubs could be relegated purely on a calculation, to be replaced by Championship clubs who’d not cemented a promotion position on the pitch.
The FA is merely trying to ensure that the PL actually gets played out, hence their stance on Monday, should that become impossible due to circumstances, then their position on this would alter, as it’d have to, as the PL would never sanction it, and rightly so.