In terms of the rearranged fixture, it could be as late as Friday May 12th. Clubs have to have their players available for four days after the end of the normal season:
"29.3.2 *any Registered Players,
remain available to the Club during the period of four days commencing with the day after the day of the Club’s last fixture in the League Competition (including play-off matches) so as to enable the fulfilment of any fixtures (in full or in part) ordered by The League to be played after the end of the Season (if any)."
New one on me.