That's all assuming the Viking Age was caused solely by economic pressures and greed. There's substantial evidence of trade across the North Sea and in the Baltic; it's not unreasonable to assume there was also trade going to and from Frisia and the Rhineland especially since their ships were capable of going up rivers.
A bit more complicated than that, before the conquest of Frisia by the Peppinids, Frisians had the control of the North Sea trade between England (see Bede at this subject), Christian Europe and Scandinavia. The northerners themselves didn't participated directly to this trade before the conquest of Frisia. Then, they sort of took the place of Frisian, let emptyby the Franks.
Any of the merchants trading with Christian kingdoms would have faced heavy duties simply because they weren't Christian.
You seems to exaggerate at the big scale the Christian/Non-Christian divide regarding taxes. I would only give the exemple of the Magalona's harbour (not the old one, the "new") in Mediterranea where Muslims merchants were welcomed around 800 (while they were still piracing the sea and raiding the coasts).
For the heavy duties, considering how much the trade with Northerners was made in their trade centers (no trade center worth of mention in Carolingian northern Germany), i'm pretty sure that if it would have been a discrimination based on religion then it wouldn't be the Christian that would have enforced it.
Also keep in mind the first continental raids happened in the wake of Charlemagne's conquest and slaughter of the Saxons and the Frisians.
No they didn't. Raid in Carolingian Europe happened 30 years after the conquest of Saxony, and 100 years after the first part of the conquest of Frisia.
There's some theories the destruction of the Frisian fleets effectively popped the cork on the bottle opening up the region for larger Viking raids.
These theories are not only unproven, but they base themselves mainly on the attack of Carolingian palaces. It of course not explaining at all why England was the first touched, at the contrary of the economic explanation that states Scandinavians took the place of Frisians in this part of the world, and tried to take the continental "market" as well.
Another possible element, keeping in mind the active missionary efforts in period and the economic pressure being placed on the non-Christian Scandinavian merchants, could have been religious or social in nature.
Actually the missionary efforts greatly helped the integration of early scandinavian in European "theater". Technically, when a king or a chief welcomed a missionary, even if not converting himself, he gained some diplomatic importance for christian kings and that could been of a great help sometimes (see the help given by Louis for Danemark)
Regarding the "slaughter" part, not saying that it didn't happened but that our historic perspective suffered a lot from the medieval black legend. Remember that the speciality of Frisians and Saxons was to raid periodically Francia, since the VII century and they weren't known at being particularly kind.