1. can I store the main output level
2. in the edit>mix>post menu, the post mix parameter sets the relation between dry analog signal (yellow marked below) and the digital wet signal, right?
No. It sets the mix before the D/A.
3.a. what does the post level in this same menu do? is then it just influences the whole digital signal path, right?
Yes and yes.
Also, please, one question per line.
3.b. I needed this parameter after the analog and digital signal is merged together for real master volume in the end? how could I achieve that?
You have to use the FX Loop mode. I don't know of any other way. this the red marked post level below?