I made sure to check if this had already been answered but I could not find one. So my question is: is なんだから related to のだ? Looking at some example sentences it seems to imply something like that but I'm not sure. I can't see to find a definitive answer.
...なんだから is related to のだ through various processes at work here. There is a simple contraction here, i.e. なんだから -> なのだから.
Let's split this phrase up into the individual bits: な+のだ+から.
The な is required before using のだ when the preceeding work is a noun or na-adjective; in other words, in sentences that would end in です (だ). This is similar to how sentences that end with ~と思う require the subphrase to be marked with the copula (i.e. ~だと思います).
Hopefully this will help clear up some ambiguity.