Got a few issues I'm wondering about here.
There is a presidential election scheduled for 25 May, but what is going to happen with Parliamentary elections?
My understanding is that, at least in the last elections, Ukraine uses a mixed voting system, with half the representatives being elected on a majority constituency basis and half appointed from party lists on a proportional basis.
It looks like ~12 seats (going from bad maps, looks like somewhere between 10 and 13) have just attempted to secede.
Ukraine can't recognise this. There is no legal basis to suggest that the secession is legitimate. At the same time, any constitutional or other major action by this provisional government is going to be questionable and weaken their (correct) arguments about what's going on in Crimea being illegal.
That means, in my eyes, that they have to, somehow, hold elections that include Crimea. That looks flatly impossible. But I'm not clear what the clean solution to this is. They can't exclude Crimea while holding legitimate elections.
As far the secession, if I were a Crimean secessionist who wanted to join Russia, this would be the worst possible method. It's ensured that the region is stuck in a 'disputed' status into the foreseeable future. I can't imagine more than a handful of nations recognising Crimea being part of Russia, and none with internal separatist movements. Hell, China have had to take a very serious stance of pretending the whole thing doesn't exist, given it strikes scarily close to home to some of their own separatist regions. The way the vote was conducted, under the apparent shadow of the Russian military in a rush, means it is unlikely to be considered legitimate, let alone legal.