I don't think any of that is particularly relevant to my question though. I'll elaborate a bit.
NI was a huge painpoint in the Brexit negotiations, to make sure it stayed properly within the UK, the Good Friday agreement could continue, etc. That's an important part of the reason why part 1 (Brexit) of the negotiations took forever. What we are discussing here, is that the UK government for the trade agreement negotiations (part 2) now apparently wants to renege on some of the stuff that was agreed upon in part 1, including the situation with NI. That was unthinkable previously, which one might have thought was because of a will to keep NI fully integrated in the UK; but given the apparent change of heart now regarding that clause in the Brexit agreement, it seems the previous concern with NI was due to the need for DUP support for the government, not because of an actual interest in the region - regardless of the percentage of people in NI that voted in favour of Brexit.
As for Scotland: a no-deal Brexit (or similar) goes against what a strong majority over there would like to see given the best scenario (again: regardless of who actually voted for it), and will fuel talk of secession.
You're right that the majority of the UK's population live in England + Wales, and I am not all disputing that the UK government can do what it wants and has the mandate for that. I am just asking how much they care about ensuring NI and Scotland stay happy and won't want to secede. The answer seems to be 'not much'.
Obviously, the idea is that, if you take these people out of Wales, the remaining population would have a remain majority. Not that ONLY these retirees voted leave.