Thank you for taking the time to write such a thorough report!
The Training Hall showing up with a Stage 0 message and getting stuck in that state through multiple match starts sounds super annoying, I'm going to try to recreate that!
You mentioned thinking about adjusting mouse sensitivity. Did you happen to do the auto-calibrate that pops up on first launch? If it was still hard to swing after that, I'll take another look at the baseline target for mouse sensitivity (the calibration is because OS, hardware, and browser all seem to require a different setting).
Thanks again for taking the time to play and document everything so carefully. It's a huge help for improving the game!