1) Statebuilding among both Scandinavians and the Germanic tribes was tied to economic growth and centralization among their neighbors (Franks and the Romans, respectively)
Depends on your definition of "neighbours". It's almost certain that fall of WRE had an impact in Scandinavia, with "balkanisation" and militarisation of tribal entities, coupled with a general pauperisation (which saw, among other things, the predominance of fortifications and axe-warfare).
Similarly, far-range trade did much to reinforce Scandinavian polities in the VIIIth century (and to provoke a wave of raids in the IXth, when this long range trade was declining suddenly with the Abbassid economical decline).
Entities that, while not neighbouring, had a capacity of trade or even political projection could play an as much important role than peoples "next door".
Therefore, even if the Roman Empire survived, at some point, you would see Scandinavian raiders heading against Britannia, Gaul, and Germania.
Not necessarily : the shift of trade roads in Early Middle Ages (at the benefit of North Sea trade roads) may simply not appear as such ITTL, and still favouring the Rhine/Rhone axe, with trade centers appearing on Frisia, controlled by Rome.
Basically, you may have more chance politically speaking, if you have still North Sea system flourishing in similar times, to see a Roman variant of Frisian trade network; Scandinavian getting connected trough it (and without Volga trade road).
Causes of the mass raiding were quite tied up to more or less immediate reasons (Frankish pressure, collapse of trade networks, sheer opportunism on a much troubled Carolingia and AS England) for that their exact consequences may not appear in a widely different geopolitical context.
For exemple, Slavonic principalties didn't began going all raiding on Scandinavians and Germans right after their structuration trough Frankish influence; but beneficied from a larger, favourable geopolitical situation (absence of Scandinavian rivalty, for exemple).
Eventually, I'd tend to think that mass raiding is more likely to happen in a destructurated context (see Somalia) than structurating.
That said, yes. Scandinavians would have too much of a favourable position for they won't engage in naval coastal plundering, even if it may be even less distinguishable from "forced trade" (as Greeks, Portuguese, etc. practiced in their time)
