ice--I think you have overlooked a few points. First, once you form an alliance, it is not that easy to just switch. Ethan formed one with Lex and Tom by the end of the first few days. He has even said in the last episode that the 2 people he was closest to personally went to Samburu. You don't just switch alliances at a whim like that because MB shook up the teams.
My second point is more strategic and is a combination of things I have said and others have said about Ethan's position right now. We cannot know for sure whether Ethan was ready to dump Clarence. But after the threat of a delayed merge there is NO way he can do it now. Next Boran TC, Teresa has to go. Otherwise Ethan is at risk of a 2-2 tie if they go back to TC in E7 (a time that should be after the merge but Jeff threatened may not) with no guarantee of winning the tie breaker (because Jeff did not announce Silas' vote and there is no guarantee who Clarence would vote for in E6 or whether it would be announced). Thus, Ethan has to stick with Old Boran and cannot join with Frank and Teresa (unless somehow Boran does not go back to TC prior to the merge).