For Czechia, small corridors from the Sudetenland to the islands are an option - when the islands are that small and the national borders so "zoomed in", it's easy to make the case for a touch of border gore (especially when one nation is so much more powerful than the other). In some cases though, the islands are too far in for that to work, and so for those, exclaves or population exchanges become the only choice, assuming they can't just be left alone. But it isn't a lot of land, so the small corridors (which can easily be settled with Germans) may work.
For Romania though, if Hungary wants the Székelys within their borders then they'll either need to settle for an exclave (not happening) or go for borders similar to the 2nd Vienna Award IRL via war or diplomacy (and all that entails for the new Romanian population, functionally reversing the problem). I doubt a long corridor would work. The Ukrainian and other minorities present in Bessarabia might fall under the same idea, but I doubt those sorts of negotiations would occur between the USSR and Romania.