After my session, I was unscrewing the mouthpiece when only the screw-on part of the mouthpiece part got stuck there and the rest of the mouthpiece came off without it. I had to to unscrew it with a plier.
Howdy, welcome to the Forum and all the fun. Sorry you and your DV are off to a rocky start, I hope it'll get better and you'll enjoy it as most of us do.
If I read right it sounds like the battery life is low, hopefully a few cycles will sort that out. There are 3 batteries in there, all in series, all it can do it overcharge the strong ones a little bit to get the weak one up to snuff. This is the case where long charge sessions are the best advice I think, normally you get best battery lifespan from stopping just short of fully charged, but for now you need to overcharge a bit.....
The MP sticking is a common enough thing to happen. The general cause is thermal. Either condensed vapor glues it in (doesn't take much....) or when it's hot (expanded) it gets tightened a bit more so when it cools (shrinks again) it's good and stuck. The 'fix' is the same both times, heat it up before trying to unscrew. Even if it's stuck hot, a few 'hot/cold' cycles can break it loose again....obviously late advice, but next time you know.
I think you should contact the maker, early on I recall him replacing such early failures (lots of guys use it for a handle a time or two.....).
Anyway, neat little vape for the most part, hope you're enjoying it fully soon.
OF