As quilters I guess we need to follow the carpenters rule.
Measure twice cut once. Everyone should be sure to square up pieces before mailing them out. Sorry this has happened.

I don't know anything about round robin groups; but can't you send it back to the offending "birds" and ask them to fix their mistake? In my experience, if you try to fix a wonky problem later it just emphasizes the wonkiness, if that is a word.

I can't believe someone used pre printed blocks in a Round Robin quilt. I know I would be very disappointed to see that in a quilt that I trusted to others to use some thought before just adding something. I would include a note before sending it off to the next person what you had to do because the printed blocks are not accurate.

I would put a small row of nine patches or 2 square patches around your added border which should be where you adjust the error. It will work just fine.