I
The answer is very simple: Paganism, at least the European one, has a concept that can be summed up as "if the enemy wins despite all the odds and we suffer disasters all the time then it means that the enemy's gods are more powerful than ours. Let's convert right now!"
So, this means that Germanic Paganism becomes dominant instead, maybe influenced by religious philosophies developed by the Graeco-roman pagans, maybe not. Without the Church around to keep knowledge around, I think that the Dark Ages may get even more Dark, but in the sense of begin even more obscure than they are now. In fact,I'd dare to say that there would be a full-on technological regression to the second century Before common Era.
Er, no. That is not how Greco-Roman pagans perceived things. They would percieve setbacks like that as being due either to natural forces or to their gods being disrespected or offended in some way. They would not decide, 'oh, these guys' gods are stronger, lets abandon thousands of years of tradition for the ways of foreigners, instead of just incorporating those foreign gods and rites into our own system'. What you would get would be syncretism(sp?) wherein Germanic gods are identified with/as Mediterranean gods or absorbed into the pantheon in their own right.
Mostly you seem to be under the misconception that polytheistic religions are mutually exclusive in a similar manner to Abrahamic faiths. They were and are not. One could, and some do, worship multiple pantheons of gods, or only some gods from one, or some taken from two or more, or smaller, local gods. Overall, religiously, you would end up with gradations, in heavily Germanic areas you'd get worship of Germanic gods, in areas with significant mixing of Germanic and Romantic peoples you'd get a composite religion including gods and myths from both sides, as had already previously happened with the Roman conquests of Celtic lands, and in places without any significant Germanic settlement, religious practices would not change.
I am not being terribly eloquent, and I appologise for that, but the gist I am trying to communicate is that Mediterranean Polytheism was an incredibly flexible religion, being already composed of many older belief systems, and would not have been overthrown or particularly disturbed by the introduction of Germanic paganism to the mix, for neither was exclusive in the same manner as Christianity.
On the matter of technological decline, I rather doubt it would be as bad as you posit. There were centres of learning and scholarship before Christianity, and I rather doubt that the scholastic tradition of the Roman world would die out without the Church. Especially as most of the 'barbarians' admired Roman culture and often attempted to emulate it.