simple solution to the end of this season and also next season.
Defer the end of season to whenever, and when it is possible to play the season out - allow it to end in the normal way - all clubs signed up to the same rules at the start of the season and those rules should apply right up to the end.
Allow promotion and relegation to follow the normal course.
Obviously under the current circumstances this could well eat into next season.
The next season :-
1. Split every division into 4 groups (depending on seedings) of equal number depending on how many are in the division.
2. Allow each group to play out a mini round robin set of fixtures.
3. All the 4 group winners play off in a s-final 3rd place and final knockout for 1st 2nd 3rd and 4th places
4. All subsequent groups to follow suit for end of season league positions.
5. The bottom of each of the 4 groups playing for relegation/survival.
6. Relegation and promotion to follow the natural course having been decided by the appropriate knockout matches.
7. Knockout cup matches to be played on a one match basis throughout the mini group season.
What the above does is (a) condense the season into 12 or so weeks, giving time to prepare for the Euro 2021 and (b) maintains the integrity of the Leagues because everyone is playing to the same rules from start to finish.
Not an ideal solution, but a solution that maintains competition and integrity of the prem/efl/national leagues etc.