If Sarah and Leah did Yibum (to other brothers), and the Yevamim die - they may not get married; R. Elazar says, since they were permitted to the Yevamim, they are permitted to others.
According to TANA KANA there is a solution: they will divorce after the yibum (not with kohen) and will marry again. Does the gemara encourage solutions like that?
You seem to be suggesting that (according to the Tana Kama) there is a way for the woman to marry out (i.e., not the Yavam but any person she wants) and to avoid becoming Asur because their Tzarah's testimony is unreliable, by first marrying the Yavam, and the getting divorced from him.
Yes, of course that is true. In fact, she can also avoid the Isur by simply getting Chalitzah, instead of Yibum, from the Yavam. (Assuming, of course, that she can convince the Yavam to do Chalitzah or to give her a Get.) There is nothing wrong with that solution. In fact, the Mishnah's Isur only applies in the relatively rare case that both Tzaros did Yibum to two brothers of their husbands, and later both of those brothers happened to die childless. In short, the wives probably wouldn't even consider the possibility that this strange series of events might occur.
Mordecai Kornfeld