That "sagging line" thing gets so many of us and it's maddening because you trust the line more than your own eyes. String does stretch once it heats up, and once it's cooked it never really goes back true. A fresh line is cheaper than redoing a course, so I'd just swap it out.

The temp of the jug matters more than thinning it in my experience. I keep mine inside overnight and that alone makes it flow way better than the cold stuff sitting in the garage all winter.

Oh man, we got burned the same way! We dropped $300 on a big inflatable slide for my daughter's birthday and the blower motor started making this awful grinding noise about 90 minutes in. The company sent someone out who basically said it was our fault for running it too long, which is nuts because it's meant to run all day. They still charged us the full rental plus a trip fee and acted like they were doing us a favor. The whole thing left a bad taste, and honestly the kids were over it after the first hour anyway. Buying used and selling it later sounds like way less of a rip off, plus you're not stuck paying some guy to show up late and then blame you for everything.