I can't think of any situation where Westminster will prefer for Scotland or NI to secede. You'd simply be weakening yourself politically and economically, throwing away tax revenue, and looking pretty incompetent (even more so than now).

From the EU perspective, my own view is that if Sturgeon can find a way to persuade Scotland to go the full EU i.e. Eurozone, no opt outs and no rebate then I can definitely see Juncker finding a way to accommodate by doing some sort of quid pro quo with Spain. Whether the Eurozone economy will recover sufficiently (and/or the UK economy will tank sufficiently) to make that politically viable is anyone's guess.

For NI, if it wasn't for the super strong unionist movement, I'd say that there was a lot to be said for them going full EU: they're small, relatively poor, bordering Eire. But that's a really, really big if, probably enough to kill any real chance of it occurring in the next 20 years.