So, wait, they're dealing with some Ukranian national government and hope they stay in power?
They're doomed to fail in that one, you do realize?
What your scenario requires is an even more dramatic collapse of the Russians (who start out with a better position this time around), totally pro-Ottoman Anglofrench (who would also be, likely, stronger this time around), and a Ukraine that can be propped up.
Please forgive me if I think "if not extremely likely" is a little too enthusiastic.
In answer to your previous, there could be an interest in the large Turkish population that lives there. In this period, Crimean Tatar is virtually interchangeable with Ottoman Turkish. I think you'd find great support for an Ottoman occupation among the population - about 1M Tatars.
Ukraine is doomed on its own, but not necessarily if it is backed by the Powers. The Ottomans would have to do all the heavy lifting, but I don't think the Ukraine is quite as hopeless as you do, and I think you may be overestimating the ability of the Red Army to do too much about it at the conclusion of the war. Russia is devastated and exhausted, and the Red Army is not really that big or powerful. The Ottoman army in 1918 or so, not having to have fought for four years, will pretty badly outmatch it, although that won't happen in the Crimea without a lot of Entente logistical help.
The Reds were willing to make peace with Enver on his own in Central Asia (to buy time) - the Crimea and/or Ukraine stiffened by a few Ottoman divisions is probably not beatable by the Reds for a few years. If that's not enough time for the Ukraine to stabilize, then there would be a big problem. But even then, is the Ukraine THAT important to the Soviets?
IF the right diplomatic situation exists, I don't think it's that far-fetched. To summarize, it requires:
- A Russian collapse,
- such that the Black Sea Fleet is neutralized.
- Independent Ukraine
So far not too big a stretch. All these happened in OTL.
- Entente willingness to back an Ottoman move into the Crimea and support for an independent Ukraine.
- Logistical help for same
I'm really not sure how bad fear of communism was in 1918. If it was bad, this is conceivable. The Entente powers
did launch some pretty serious interventions on behalf of White forces in OTL.
It seems to me that Britain and France are going to have a problem with the Ottomans extending their power so widely, but with the exception of Baku, there's not really much involved that are of particular interest to anyone. If the Ottomans are able to positions themselves as allies, it might be considered desirable to extend the Ottoman Empire, in which Britain and France would be fairly certain of having a dominant economic position.
So I think "unlikely" is pessimistic enough.