I have no idea if all Taiwan models have the OEM unlock issue. And I have no idea what the second batch of phones are from never-msrp, I avoided that seller so I have no info on them other than what I have seen posted by others.
Flash firmware? I'm not sure what you mean exactly, flashing firmware to try & resolve the OEM unlock issue doesn't work, it's been tried already, it's not a firmware issue. Or if you meant flashing firmware to resolve the missing band issue on the N, that wouldn't work either, that's a hardware difference.
Samsung Pay, no idea, I've got TWRP & root. Haven't kept up with what workarounds there are these days if any, I just remember tripping Knox breaks it & I have obviously tripped it. I have never used Samsung or Android Pay ever, never had a stock Android either. I don't care for the feature & probably never will. I don't use credit cards unless I purchase something online.
Sorry I can't be of more help, I don't even have the OEM unlock issue. I have the F from Clove UK. I'm just here in this thread cause I'm curious & hopefully see a solution.