If I purchase a day or 2 of the Tesla toolbox. Can I rest the error codes and use it another week or so? Will it tell me what is actually wrong with the battery pack? Maybe it's something that I can repair or replace.

 

Toolbox doesn't talk directly to BMS, it can only send it commands. One of them does involved resetting the isolation (failure) counter, but I have an educated guess that could make things worse.

Some of the service managers at Tesla Service Centers really want to keep old Teslas on the road and running. So sometimes whining a little bit about the cost can actually get them to discount it a bit. It works even better if you had the bad luck of it failing just barely outside of warranty, or it would be under warranty if not for some technicality.