Yup, any UN mission will be a lame duck since neither side is going to agree to a Peace-enforcing mission.
Ukraine might have originally. They were calling for one from the start. Now, however, I agree; they might have difficulty trusting one.
By the bye, the talk about how the US and West would accept this coming from a pro-West Russia is amusing, because it reminded me of the strategic assessments of multiple countries in the last century alone: Romania's expedition against Communist Hungary (suppressed), Italy's expedition against Fiume (suppressed), Hussein's expedition against Kuwait (Gulf War), Turkey against Cyprus (embargo against the filibuster state), Indonesia against Timor (diplomatically opposed), and Indonesia against both Brunei and Malaysia (militarily opposed). In fact, the only ones I can think of that that could be considered such were India in Goa and Indonesia in West New Guinea, both of which took on a strongly-ideological cast in the anti-imperial, anti-colonialist United States, and Morocco in Western Sahara, which has largely been left by all third parties (both the USA and USSR, and later Russia) to the UN to (fail to) resolve. Israel may be considered a possibility as well, but it's been very careful not to officially annex the lands it took from Syria in part to avoid igniting an unnecessary political powderkeg, and it did ultimately return the lands it took from Egypt as part of a permanent peace agreement.
Plus, let's assume that it was accepted; that would certainly be a triumph of realpolitik, but it does not follow that the rejection of an annexation is also realpolitik. That is to say, the West might hold their nose if other political contingencies preclude a full response, but that does not mean they would be pleased to do so. Nor does it follow that the converse is true that they would have rejected it automatically even under the circumstances of a legal, properly-conducted referendum being held under independent, third-party (say, the OSCE) observation. The point of a proper referendum in the region is not that it would have been legal or illegal (guess how many complaints have been brought up by the US State Department about the "illegal" Catalonian referendum in 2014), but whether it can be said to have accurately reflected the opinions of the people in the region. The referendum that was held, if it does, does so only by coincidence; it was intended to have a particular result, and matters were arranged accordingly. It seems odd to censure America and the West to act according to their moral beliefs regarding the matter, and believe they should instead ignore their morality entirely to act according to realpolitik alone (which would be to quietly brush the Crimea annexation under a rug and maintain economic partnership with Russia), while praising Germany for taking a principled, moral stance in other matters.
Oh, and by the bye, fun fact to those arguing that America has "always opposed" Russia: did you know that America's closest friend in Europe was originally not Britain, and only barely France (Jeffersonian politics aside, America rarely had the stomach for Bonapartism), but actually, consistently Tsarist Russia? It was always a very odd friendship between the strongly authoritarian and democratic nations, as odd as the Franco-Russian Entente would be later, but it was one that existed nonetheless. Russo-American animosity, in terms of the relative history of both nations, is actually quite young, clocking in at around a century.