I want to do everything in this game all side quests and question marks , but I know that if u go past a certain level and u do new game plus u will find urself annoyed by the fact there will be enemy and diagrams higher than lvl 100 ,

I want a simple answer and hope from ppl who have experience with this issue :

I just want to know what lvl should I not go past before starting a new game plus . Is lvl 50 safe ? I heard ppl starting at lvl 70 have major game breaking bugs .

If I make all side quests/quests/question marks would I be overleveled for new game plus and encounter this bugs ?

I am lvl 17 only from finishing main quests in velen without side quests ..... now going for the isles , wos really hoping to do some side quests but I dont want make the main quest line go grey ... this is very annoying already failed some quests and some went grey , it almost maked me hate the game but I chilled and I think il just continue with main story finish it without doing anything else untill new game plus ...

I don't know where you heard this? It sounds like nonsense to me. I've never found a diagram that was lvl 100, and afaik, the level cap for NG+ used to be 70, apparently Blood and Wine pushed it to 100 (can't verify) - at any rate, 100+ gear would make no sense whatsoever. Simple answer: You will never make it to level 70 during the vanilla playthrough. Like, seriously, unless there's some mod or bug or whatever, 50sth is probably as high as you'll ever be, and you'll only manage that with playing the expansions as well and doing most sidequests etc when they are not greyed out yet.
I have never heard about any bugs that manifest only because you are such a high level, there have been a couple of bugs related to being on NG+, however.

Advice: Just play the game, do the side quests more or less in order (starting with lowest lvl and move up) for max XP. Do not worry about some random quests getting greyed out, it's pretty unavoidable.
Remember that most all XP come from the quests, the main quest will give you the most XP, so if you just proceed with that you'll have a ton of grey sidequests, you're basically propelling yourself forward at high speed while complaining about the vertigo ;) Just, relax, and play the game in a way that's fun for you. The game does not lend itself well to a min-max approach.
Failed quests have nothing to do with level progression, btw. There aren't many that can fail, at any rate, and if so, it's because you moved the story along.