It depends on whether a system allows a unilateral separation. Can the Act of Union be repealed by Scotland without England's cooperation?

The EU allows it, although Brexit might trigger some eventual hardening of the withdrawal rules. I think the EU is an outlier among geopolitical arrangements, though.

Latterly it has been sufficient for significant civil resistance or economic cost to be sufficient for a ruling state to cede sovereignty over another region, a process Putin is currently trying to reverse.
Ukraine was kind of the opposite, no? With Yeltsin being the leading force of the breakup so he could get rid of Gorbachev.
Actually I don't think we can claim that there is a historic trend toward civil resistance leading to a ruling state ceding sovereignty. In the context of a rising USSR and the US still not being the undisputed hegemon of the western world? Sure I can see this happening. But that was India and Suez. Then came Algeria and Vietnam.

Irish unification doesn't seem that unlikely. It would be less of a headache than the status quo for basically everyone, the US would probably support it for domestic reasons and the political tendency that favors it has historically been willing to endure quite a bit of pain.
But Scotland? Who is going to force the UK to release Scotland? The EU really isn't a serious political actor and even if it was, who really wants a second Irish land border situation, but worse? The US has no reason to care. I guess Moscow would send sincere well wishes, but that and eight quid will buy you haggis. So my guess is still: The SNP will demand a referendum, the Tories will say no and that's that for the foreseeable future.