Wales, Scotland and Northern Ireland now all have nationalist first ministers

Yes for the first two since it's an unambiguous free trade and free movement of labour zone extending between areas of similar education and economic development, even if they receive more money from tax than they supply, since internal free movement is an essential prerequisite for most kinds of manufacturing to be profitable. The latter is an oddball.

It obviously benefits the UK populace to put off having to deal with any civil conflict in NI indefinitely but this is minor compared to the other side of it: does having a non-united Ireland as a way to reduce Irish competitiveness and geopolitical regional heft produce more value from preventing problems (like losses of jobs, increased corporate tax avoidance, marginally increased rates of corporate espionage by third parties using Eire as a base, Ireland having more negotiating power in commodities agreements, Ireland acting as a refuge for enemies of the state in times of UK civil conflict) from arising than it costs to maintain NI?

That latter side was a very serious issue in the history of the UK, England, etc prior to globalisation and the UK has to hedge its bets against the odds of globalisation going away. A good book on this is Land of Troubles, published by Routledge.

SF would ideally prefer to unite with the republic. Actually taking NI out of the UK in any form is guaranteed to make the unionist side of the conflict apoplectic and basically throws out the peace agreements that were achieved though. I'm not sure if Trump actually understands that there are people against a united Ireland, not least because those people would ordinarily be his supporters.