So, the real issue you are having is specifically with the UOCofUSA.
Why don't you just ask them? I know you have someone specific in mind. Why don't you just ask this priest why he joined the UOCofUSA instead of the OCA? I'm sure he'll be able to give you a better answer, than all of us guessing.
As long as those parishes remain ethnic, that the Ukrainian language is used for the parishes that are majority "ethnic", I don't see the problem.
Unless this new convert priest does something blatantly against the ethnicity of the parish...what's the problem?
Unfortunately, most second and third generation parishes, no longer have the language issue, as most speak only English. Therefore, as long as the priest is a devout Orthodox, and serves the Church honestly, with love and integrity, and doesn't teach against the ethnic roots of the parish, where's the harm?
In fact, it's kind of an honor to have non-ethnic, love an ethnicity so much, that they CHOOSE to join it.
If there are not enough Ukrainian priests to go around, would it be better to simply close that parish and call it quits? Or is it better to have an English speaking parish?
After all, the goal of the Orthodox Church is in fact to save souls, regardless what language the body housing that soul speaks.