you're confused about what i'm saying...I havent said the levels they reached didnt matter, i'm saying the names of those in between levels doesnt matter...just like you dismissed ss4 cause it wasn't cannon, so are all the names of the in between levels....those levels didn't have names, fans made that shyt up cause of character favs...the only names of levels that matter are: ss, ascended ss (ss2), and complete/perfect ss (some shyt like that) (ss3)...
we're not talking about imperfect cell though, we're talking about perfect cell...the cell vegeta wanted to fight, that's why he let him absorb all the droids...that cell was stronger than vegeta and trunks at that current time cause they weren't ss2...they were in between ss1 and ss2...that's all that part showed....
vegeta trained hard, but never harder than goku and never as long as goku...goku trained his whole life, even in death...vegeta didn't...he didn't start training like that until after the frieza saga...and even then, he didn't do it as consistent as goku cause of his pride...he always believed since he was the prince, he didn't have to do it like a common person like goku...that he should've been naturally like the legendary ss...that's why he never got as strong as goku, ever....he was always playing catch-up
also, they never slept in ss...in the chamber, they walked around and chilled in it, but never slept as ss...and that's only during the cell saga...they rarely did that regularly cause they saw no point since they could go ss with such ease...
yea, vegeta became ss2 after the cell saga, goku and gohan were already there before the end of the cell saga....so he was still playing catch-up...the only reason he caught gohan was cause he stopped training cause of goku not being there; cause of that, his power level stopped growing like that until he became mystic...
as for the mastering thing, that part plays no part in reaching higher levels like that...the only thing they had to master was raising their power levels by unleashing their power...and the hardest part of that was becoming a ss in the first place...just look at the series...it took them longer to reach ss than it did any other level....after they got to ss and mastered getting to that level, look how easily they could go ss...they were doing it in secs...as for the other levels pass ss, no one knew they existed until goku found out...he was always the first to do something, then he came back and let the others know...ex: the cell saga...they thought going ss wouldve been enough to beat anyone after frieza, until cell started whopping they ass...then goku shows up, shows them they can go to another level of ss, and the rest is history...