Good luck with the rocket. I sincerely hope it produces results.
Somebody seems to have done a sloppy job on the database. Pissing off the customers can't make commercial sense.
I'm doing the same trip on Wednesday evening, and this seems to have worked for me:
1. 48 hours before the first flight, I started the check-in process and selected a seat.
2. I then closed the window without completing the check-in.
3. 48 hours before the second flight, I started the check-in process again. My original seat selection was still there, so I could select a seat for the onward flight and complete check-in.
It's fiddly and a nuisance, but as a work-around, it mjght help someone else.