Also I don't think 'da Bog' expression is really related to 'Dazbog' as in a Slavic god. I think its a typical expression which refers to a monotheistic god just like in other languages. 'God forbid'  etc.

I don't know about that.
His very name, "Dajbog" essentially means 'The Giving God', or something along those lines.
It would make sense, then, that this god was somehow associated with perhaps, giving gifts of luck, or maybe even some kind of wealth.
So sayings such as "nek' bog da" or "dajbog" "dajbože" "da bog da" could very well be directly referring to this old Slavic Deity, whose very name is associated with giving.

In contrast to that, the God of The Bible doesn't give all that much, does he?

-[size=8pt] Petar Skok (1971), Etimologijski rječnik hrvatskoga ili srpskoga jezika, JAZU: Zagreb, volume I, page 178ff[/size]