EDIT: Jesus Christ, Culise, you should seriously consider a career in journalism. Your posts are 10/10! Just two questions though: if there weren't Russian troops in Crimea, what prevented Kyiv from reacting to its people trying to secede like it reacted to the people of Donbass trying to secede? And if the reunification of Crimea really was what most Crimeans wanted, can we really morally condemn the referendum for not being in accordance with all the European laws? After all, my argument is, and has been, that while the referendum was not perfectly free and fair, it did represent the actual desires of the majority of Crimeans.
Phew. Well, this is a tricky matter, but politically, actually, forcing Ukraine to take the first step would have actually given the "polite men" credibility as defenders of Crimea. The critical matter and key issue of the Crimean referendum and subsequent annexation is that everything followed from the initial invasion and seizure of the legislature - the referendum, the appointment of Aksynov, all of it was under the auspices of Russian forces. While there were protests before the 27th, there was no official response; in fact, the closest thing to an official response was a mandate by the interim Ministry of the Interior to
avoid hostilities and prevent any official crack-down. If, without such an invasion, the Crimean Supreme Council had still voted Mohyliov out and replaced him (likely with another Party of Regions official, and not a Russian Unity minority) and called for such a referendum, I would be much more likely to consider such a thing legitimate. Under a circumstance where the appointment and referendum were made possible under armed guard in a situation where outside observers were not permitted (especially in the case of February 27, where no independent journalists or outside communication was permitted to the point where MPs' phones were seized as they entered), significant questions cannot help but be raised regarding the legitimacy of the referendum and pro-Russian government.
But to answer the question, without the polarization and the clear increase in stakes all around by the annexation of Crimea, I actually suspect Ukrainian reaction would have been anemic and weak to any domestic popular opposition, at least at first. People who have to say "this is not a policy of weakness" are typically worried about exactly that. The harsh reaction to the Donbass was in part because the Crimea situation had demonstrated quite clearly that a hands-off policy of negotiation was never going to work.
As to the second point, and this is important, I
believe that a referendum may have succeeded. I do not know such a thing, and there is a significant amount of doubt regarding the matter. Plainly, at the least, there was sufficient doubt that "alternate steps" had to be taken to ensure a complete blow-out win for the annexation movement. It is highly improbable, however, that the 42% of Ukrainians and Crimean Tatars voted en masse for unification with Russia due to the ethnic polarization of the conflict.
miljan also asked about previous referendums. As for formal referendums post-independence, there is only the 1994 referendum, which was delayed from 1992. Independence was not requested in this referendum, only autonomy, and that did enjoy popular support. Subsequent polls over the last decade, however, have already been brought up regarding the matter, for more recent data.