I'm fairly sure they won't be available for us to use, no matter when the German season finishes (their registration will be for St Pauli through the end of the <UEFA 2019-20> season).
No, the loans end at a set date, not at end of a competition and no concept like the "UEFA 2019-20 season", except for continental play.
If the loan deals are to continue, both of the clubs and the player need to agree to it before June 30 (when most loan deals end).
I dont know if that would make them available though (not sure about the details of PL registration rules). Either way I doubt any of them would get any minutes this season (unless there is a massive injury crisis), it would be quite odd to just throw them into the fire like that in a presumably very pressured situation.