See, now you are trying to make up your own definition for the word universe, when a perfectly good definition already exists.
I don't think that's the case. He's basically proposing a type of multiverse argument where God et al exist in their own place and our universe is a different place which he created separate from that. That seems like a perfectly fine definition of a universe.
It doesn't really answer the question of a first cause argument, since it just pushes the question back one level and leaves it unanswered, but it's not a particularly non-standard usage of the word "universe".
Sure. But to use that definition in the context of a discussion in which his correspondent has explicitly defined the word as meaning 'everything that exists', without explicitly providing his own definition, is an equivocation fallacy.
I am talking about 'everything that exists'; He wants to talk about a subset of that, and pretend that his argument is responsive to mine.
As I am talking explicitly about everything that exists, including any Gods that exist, and arguing that nothing that exists can be the cause of everything that exists, it is fucking stupid to then say 'but what if something exists in a separate category from the universe?' - it's impossible for that to be the case using my definition of 'universe'; and it's not addressing my argument at all if he is using a different definition of 'universe'.
In either case, it's not a rebuttal; Just some babbling that he hopes might sound like a rebuttal as long as he doesn't have to think too hard about it.
So, to sum up, it sounds to me like you’re saying
“We should use this definition”
“No, I’m using this other definition”
“Screw you, you’re going to make your argument using my definition”