Would the EU have stepped up and admitted that they considered themselves to be "stakeholders"?
I seriously doubt you can find a loophole in the Ukrainian Constitution that would permit the referendum, I'd be very surprised if you could. OTHO, I seriously doubt the Crimea considered themselves to be limited by the Ukrainian Constitution as opposed to their own definition of "autonomy". A definition they obviously think includes the right to break away from the Ukraine.
It's a case of theory vs. "Facts on the Ground" as the saying now goes. Back in my day, it was theory vs. who actually controlled the countryside when there wasn't a US grunt standing on it and in spite of the briefings in Saigon, we all knew the SVN government rarely controlled the ground a US grunt wasn't at least within sight of.
The Ukraine wasn't prepared to deal with even a minor guerrilla war in the Crimea even if Russia were only matching the EU small arms supplies to the Ukraine bullet for bullet. Given that obvious weakness, once nationalists in the Ukraine got vocal, IMHO, it was inevitable the crowd who want to be part of Russia rather than the Ukraine were going to become very active. Anything, and I mean anything, that Russia felt would threaten security around their bases in the Crimea was going to trigger exactly what we've seen.
If the EU had tried to stop the mass demonstrations and get the groups "to the table" to negotiate it wouldn't have been so painfully obvious that the EU was going to do whatever they had to do in order to keep the Ukraine from not doing what the EU wanted done. That may not have changed a thing but it at least wouldn't have been rubbing Russia's nose in the fact that the EU didn't mind supporting an overthrow of the government if that's what it took to put the Ukraine under the thumb of the EU.
As far as I can tell, Russia had no reason to believe that if the EU backed new government took charge that they wouldn't be faced with all sorts of "re-negotiations" related to their bases. Why people think Russia would believe that the same people who so recently have overthrown a number of governments would play fair with them once they installed a new government, I don't know. I do know that all my Russian friends were convinced that if the EU backed government took charge and the Crimea remained part of the Ukraine that the new government would want Russia out of the bases.
Is that paranoia on their part? Dunno, but it's how they were looking at things and everyone in the West seems to have not cared how Russia would see the changes as a threat. It was a, "Screw Russia" attitude from the start that I think led to Russia deciding they should move and do whatever it takes to regain control of the Crimea.
Would they have done so no matter what? I'm not so sure they would have if the the EU hadn't made it crystal clear Russia couldn't make any kind of deal with the Ukraine that the EU wasn't going to undermine by hook or by crook.
I am not sure if they are, but Ukraine surely is since it's its own territory being carved up.
I seriously doubt you can find a loophole in the Ukrainian Constitution that would permit the referendum
From the Report:
15. It is therefore clear that the Ukrainian Constitution prohibits any local referendum which would alter the territory of Ukraine and that the decision to call a local referendum in Crimea is not covered by the authority devolved to the authorities of the Autonomous Republic of Crimea by virtue of Article 138 of the Ukrainian Constitution.[...]
18. This does not mean that the notion of self-determination would be alien to European constitutional law. However, in its Report on “Self-determination and secession in constitutional law” quoted above, the Venice Commission concludes that self-determination is understood primarily as internal self-determination within the framework of the existing borders and not as external self-determination through secession.
So you are correct, there is no way for Crimea to secede legally under the present constitution. Which is, kind of the point.
It's a case of theory vs. "Facts on the Ground"
Yes it is. We have a war. soon there will be a larger war. With this post I simply wanted to establish what the legal facts are.