Ancient Carthage is an example of a historical noble republic of "reformed" pagans.
I can see why merchant republic doesn't quite fit.. but why would you say they were reformed pagans? What ties them closer to the already reformed religions of the game then it does the unreformed ones?
Didn't vikings actually do a whole lot of trade with some groups like the muslims in that time?I mean, you say you're civilized enough to be a merchant, but how do you convince your horde to stop pillaging and trade instead?
Merchant republics still seem like an easy switch for them to me.