What would you do ;)

[fen01]

Cut it 15' from the bottom. You can't fall so far then😁 better job all round...

[TGB]

15' isn't much. Are you sure they know how little 15' off the top will be. The kit being of unknown history is a no no. Don't climb with kit, the history you/they know nothing about.

 

As to the 6mm prussiks. For aided rock climbing purposes, 6mm is pretty standard. 5mm isn't enough and 8mm is both awkward to handle and needless weight. As a tree climber, you'll be used to a lot more carried weight. As a rock climber, I'd be thinking, "New chalk ball, more chalk = more weight. Old chalk ball = may run out of chalk but less weight. (Difference is a couple of ounces here.) Hell! I don't want that weight. The new ball can wait for another day."