OK, we have a date for the rematch. It will take place just before Christmas, on 21st December:
https://talksport.com/sport/1899340/tyson-fury-vs-oleksandr-usyk-new-rematch-date-confirmed/credit to JollyGood for posting this on BTT
Usyk almost definitely will not be stripped before 2nd June, but that's not to say the IBF cannot do it after that date.
Given the rematch is scheduled 2 months later than initially indicated, any potential mandatory defense will not take place until around a year from now at the earliest. I find it hard to believe the IBF will grant the exception and postpone the defense for that long, as it would be unfair to the challengers. Plus, there's no guarantee the winner of the rematch will be interested in defending that belt at all.
I expect there will be no interim belt on the line on Saturday and Usyk to be stripped after the 2nd of June. Then the fight between Hrgovic/Dubois and Anthony Joshua would be for the proper belt (not the interim one).
I don't see much sense in introducing an interim belt given Usyk is not seriously injured and would be ready to fight if he wanted to.