...as far as I can see, the decision to migrate seems to be up to five
or so ELU users...
It takes five close-votes to put a question on hold unless a moderator is involved with a single hammer. However, it doesn't necessarily mean it takes all five close-votes to migrate it. Sometimes, it takes only 3 close-votes to migrate when the other two votes are cast for other reasons such as 'general reference' or 'proof-reading'.
Is there a list of migrated questions?
You can search using migrated:1 to see the list of questions migrated to and from ELU.
When you reach 10,000 reputation points, you will be able to use the moderator tool > review > migrated.
That seems to make migration hardly revocable, although (as I said) it
is decided by only a few users.
If a question is closed on the site to which it was migrated, the question will return to ELU.
I have seen one case where @deadrat posted a request on ELL Meta to migrate a question back to ELU by closing the question on ELL. It was done within an hour or so and I don't think "migration" is hardly revocable even though it will take some time and efforts to do that. If it is coordinated well between moderators of each site, it would be even faster and easier.
There are many questions related with "migration to ELL" which might interest you.