Scotland is in GB but NI isn’t as it’s on a different island. Both are part of the UK.
DUP is the biggest party, but by “mainstream” I mean “not batshit insane.” The longtime largest party, who has since been relegated to #3 or #4, is the Ulster Unionist Party who didn’t support Brexit.
I agree with your analysis that the EU will require border controls in Ireland (unless the UK negotiates participation in the single market, which looks tricky). But I’d add to it that the UK will require border controls if they wish to “regain control of immigration”, which I think is one of the state objectives of most Brexiters. While the RoI doesn’t participate in the Schengen area, there is a right of free movement throughout the EU. If people can travel from anywhere in the EU to the RoI, and then cross the border into NI freely, that pretty much puts the kibosh on any attempt to control immigration into the UK. (Unless they are going to put intra-UK movement controls between NI and GB, which nobody has suggested, and which I think would be politically unacceptable.)
Also, there are a lot of English in Wales. The vote is broken down by geography, not ethnicity. From the people I’ve talked to, the ethnic Welsh voted “Remain”—not 100%. A higher percent for Welsh-speaking Welsh, but again not 100%.