Hi! I can talk a little bit about the playtesting and balancing aspect. For the team, I set a deadline of the of the day on that Friday for us to have a decent, playable build on itch, in order to leave us about 1.5 days for playtesting and polishing. The valuable playtesting mainly came from the non-programming team members, so basically just the people who were working on the game but hadn't actually played or seen it before. In addition to that, I forced every level in the game to go through multiple revisions, playtesting and providing feedback myself starting on the very first day.
As for the teamwork part, I can provide a little bit more detail. One important thing we had was a requirement for all programmers actively working on the game to be in one discord VC at all times. This helped facilitate the aforementioned claim system, since it was as simple as asking something like, "hey is anyone working on level select right now?" "no" "ok then no one touch level select." Despite all that, we certainly had merge issues. Most were pretty benign, some resulted in people having to redo work, and some just straight up deleted entire levels (those were very unfortunate, we had to pause all work and revert commits). On the last day, we didn't actually have much to put together since we had been assembling the game as we went (one huge help is that the game is levels-based meaning it's really easy to split up files and avoid conflicts). In fact, the game had been pretty much assembled since its first itch build (on the Friday night). Instead, what we spent the last day doing was having everyone do all the polish we could. Most of the team worked on doing final level touchups and decorations (and one person did SFX) while MrPuffyWuff and I (the experienced programmers in the group) worked on everything else the game needed to be done (work was allocated like this because we didn't want one of the inexperienced programmers to make a mistake on the last day and break everything). Specifically, on the last day leading up to the deadline, I made the ending screen, player death animation, and a few various other things (such as the sand falling sound effect getting louder the less time you have), and MrPuffyWuff made the intro cutscene and reverted a buggy commit which deleted 3 levels (he was up late the night before, I was not). So yeah, we generally try to avoid only putting the game together at the end and instead we put it together as we go. Anyway oops this a really long reply lol.