The Gemara points out a stira in the sugya of shchooni gavai and Nichsei d'Var Sisin in what Rava holds. The Gemara says there is no stira because Rava goes with whoever is muchzak at that point.
So Rava is saying if it's going to be chal a safeik or not, it depends. If the lokeach is in the karka, and now you want to be m'arer his chazakah/pasul his chazakah, to take away the karka, then you are not neeman. But if the mocher is in the karka, to be m'arer a chazakah, that Rava is mekabel the taina, and that's nichsei bar sisin.
My question is why does the muchzakos help? If the mareh kama's taina creates a safeik, so we say b'chezkas baileha omedes and the mareh kama gets it. And if the taina of the mareh kama doesn't chal a safeik, so who cares whose tafis?? Let the lokeach get it. It seems like Rava is saying that the mareh kama's taina is only chal a safeik when he is the muchzakos. And the question is, who cares that he is the muchzak?? Why does the muchzakos of the mareh kama create a neemanus ha'taina?
Menachem Zaman, Los Angeles
Excellent question!
I see what might be the beginning of an answer in the words of the Chazon Ish (Even ha'Ezer 67:7). He addresses one of the principles you aptly cited, namely, Karka b'Chezkas Ba'aleha Omedes. He explains that this only applies when there is no evidence in front of our eyes that contradicts the Mara Kama's claim.
He equates this with a case where a Nosen gives a gift to a Mekabel, and afterwards the Nosen wants to claim it back from the Mekabel on the grounds of some new claim that something happened to cancel the transaction. In such a case, since the gift is clearly sitting in the possession of the Mekabel, it is incumbent on the Nosen to provide proof for his claim of Bitul ha'Matanah.
Accordingly, in the case of Shechuni Gava'ei, the Chezkas Mara Kama would indeed suffice to settle the case in favor of the Mocher, if the Loke'ach was not occupying the residence. But since we see that the Loke'ach is living there now, consequently the burden of proof is on the Mocher.
Accordingly, in the case of Bei Bar Sisin, the important point is not that the Mocher is occupying the land, but rather that the Loke'ach is not occupying it.
I hope this helps!
Best wishes,
Yishai Rasowsky