I don't understand what the problem is with working out who should be promoted and relegated from the various divisions if the season doesn't continue.
PPG is definetly not the best way to work this out.
PPG's obvious flaw is the following: if you have two teams of similar abilities in a league, then the team that has predominantly played others at the lower end of division will have a higher PPG, through positive results, then a team with similar ability who has played others at the top end of division, through negative results.
This is the flaw to ending the double 'round robin' setup early which most sports federations use.
There is of course another way to work this stuff out.
Whilst complicated, Glicko-2 is the best way to sort this issue out through an adjudication of the leagues if the season can't continue.
The season, in my opinion, should be played in full. However, if a lot of money is riding on promotion and relegation, and for whatever reason the season can't continue, then shouldn't everyone be looking for the fairest way to bring the season to a close.
Glicko-2 is not perfect but it is much more reliable then using PPG, especially in this sporting environment of an unfinished double 'round robin' tournament.