First-cause, primal-cause, whatever. How do you show that such a thing is singular? It is just as possible that there are multiple acausal events/sources/whatever that started the universe as it is that a single acausal event/source/whatever did. How can we know which is actually the case?
(for simplicities sake, I'm going to shorten "event/source/whatever" to "event", but I mean the long version)
Further, how do we know that there wasn't a succession of acausal events that contributed to the formation of the universe? Events that are completely non-dependent on previous events, even though they occur chronologically later?
Again, the rest of your argument assumes the uniqueness of the primal-cause, so I'll wait to address it until after you've shown uniqueness.