Just 2 questions. How can the game continue from here?
And can i play this move?
Well , i think that Ng5 can be answered by Ndxe5!
It's a lousy line for Black. And yes, you can play 7 Ng5, White wins material, Black ends with central pawns and the Bishop pair, probably not enough compensation.
In the first diagram, 8 ...Bd6 is much better than ...Nxd4, Black needs to develop.
The whole ...Nc6 family is a stepchild to the French, it slows the natural pawn chain counterplay of ...c5. It's not to be recommended for amateur players, you need to know what you are doing to survive.
I always play 3.Be3 in the French