So there is a chance that 50+, PVV, SP and PvdD may all not reach the threshold making PvdA making gaining more seats. But that they have to return these seats after Brexit as the threshold will be lower which they will all reach as the there will 29 seats to distribute instead of 26 seats
That will not happen. There is a temporary law for the seat distribution post-Brexit. Next week, there will be a distribution of 26 seats, based on a 26-seat threshold.
After Brexit, there will not be a complete reapportionment, exactly because of the scenario of parties losing a seat after reapportionment (or because of an MEP losing his seat to a lower placed candidate because of the lowered preferential seat threshold).
Instead, there will only be an apportionment of three extra seats, based on the 29-seat threshold, but taking the 26 already apportioned seats in account.