MB: You ask why it's legitimate to end the regress with God, rather than asking "What caused God?" You didn't actually ask that question in that post but you've asked it before in those terms I think. The implication is "Every effect has a cause except for God which is the first cause" treats God as a special case.
It's a separate question from your question about being and nothingness, so perhaps it is a digression from the part you were interested in, as you say. But I think NotReady's answer about necessity vs contingency is an answer to that question: "Why is it not valid to ask what caused God?" And my point was that as an explanation it still amounts to a kind of special pleading, but that I don't think this is necessarily a problem theologically speaking. All of theology is a "special case", as it were.
I'd take a stab at the nothingness question but I'm not sure if I've got all the context, I haven't listened to that debate yet. And I'm imagining a good bit of the difficulties will entail figuring out exactly what we mean by different words, along with the fact that it's possible Craig would speak of the existence or being of God in a special way, i.e that God's being is not like the existence of an object in the physical universe, and thus speak of there being "nothing" without really meaning that there was a state of affairs in which there was complete non-being. There's also the problem that it seems contradictory to even talk about there
being non-being. I'm sure OrP is banging his head on his desk by now reading this