Though I agree with all the general tenor of your post, I'm not so sure about one or two aspects (below):
"Obvious" to some people, but not so much to some others. I mean, there have been various all-Ireland cup competitions tried out for over half a century now and those have never caught on, with a league surely even harder to implement. While Kieran Lucid's proposal from 2 or 3 years ago died on its arse.
[The above is decidedly not a "Political-with-a-large-P" observation, btw]
Is it? (Genuine question)
Is there any history eg of cross-border cup competitions?
I always think that for a suggestion like this to have a chance, you need a positive rationale i.e. "Here's why the new league will be good", rather than a negative one i.e. "The present arrangement isn't working".
Unless something VERY drastic happens, then not a hope (imo).
Think about it. The EPL is by some way the most successful league in Europe (the world, actually). It is owned/controlled by its 20 Owner/Members. To date it has had 48 different participants from England, plus two historic members from Wales.
Why on earth would any combination of 20 suddenly cede even a couple of places to Scottish clubs, plus guarantee membership to another couple of Welsh clubs? This would just be denying the Golden Ticket to four or more of the 48.
Even the "Big Six" would be opposed to the possibility of Celtic & Rangers muscling in on their present domoination, never mind the likes of Bournemouth or Brentford, who would risk being muscled out entirely.
Beyond those examples, however, as I say, I think you have it spot on about the rest (esp Scandi/Nordic/Celtic etc).
If nothing else, barring a couple of exceptions, NI and ROI clubs simply do not possess the stadia, facilities and infrastructure to even hope to participate in a transnational league alongside other European members.