This is correct, but if you haven't fought the Vagrant of Space and Time you'll miss out on some specific dialogue as well as the boss spawn items. If this has already been done I can look into your problem further.
No, using the Debug Disk means you don't have to fight the bosses again, it'll update to the world's progress.
Sorry, I have absolutely no idea how this bug occured... As long as it isn't reoccurring, I'd just chalk it up to random circumstance.
(Translated with DeepL) Sorry, I don't understand anything but English and translations would have to come from someone else.
(Переведено с DeepL) Извините, я не понимаю ничего, кроме английского, и перевод должен быть сделан кем-то другим.
This boss is a survival test; instead of dealing damage, you just need to dodge the attacks and survive to the end.
Once you have binded the Stellar Nova key, you need to hold the Stellar Disk and press the key to open up the Stellar Nova Menu, this is where you can select Stellar Novas to use...