Reb Chaim, it is always a pleasure to hear from you with your interesting questions! You are referring to Rav Yosef Engel in Gilyonei ha'Shas on Kidushin 5b, DH Sham v'IBA"Y.

1) I think that he mentions only one source that is directly related to Maseches Demai but he cites a few other points which I think are relevant in a more indirect way. The source is from the end of the chapter 5 of Talmud Yerushalmi Demai.

The Gilyonei ha'Shas is explaining the Gemara in Kidushin 5b which says that if the Chasan gives the ring to the Kallah and says, "Harei At Mekudeshes Li," she is Mekudeshes, which means she is certainly married with no doubts, but if he gives the ring and she says the words, it does not state that she is Mekudeshes but rather there is a Safek, and there is a Rabbinical doubt whether or not she is married.

The point is that if the Gemara states "Mekudeshes" she is Vadai Mekudeshes with no doubts, while if it does not state "Mekudeshes" there could still be a Safek.

Now we go to the end of the fifth chapter of Maseches Demai in the Yerushalmi. Rebbi Bun bar Chiya says in the name of Rebbi Zeira that "they made a plantpot a Safek." The Pnei Moshe explains that this refers to a plantpot without a hole at the bottom, while a plantpot with a hole at the bottom is certainly equivalent to the ground itself. The doubt concerning a plantpot without a hole is whether or not it receives some nourishment from the earth.

The Yerushalmi now challenges Rebbi Bun from the last Mishnah in the chapter 7 of Maseches Kil'ayim which states that if a plantpot -- with a hole -- containing wheat seeds is placed in a vineyard, the plants that grow are forbidden as Kil'ayim, a crossbreed of wheat and grapes. If the plantpot has no hole, the resulting plants do not become forbidden. The question of the Yerushalmi is that there is no doubt concerning a plantpot without a hole; it is certainly not equivalent to earth, which is why the wheat in this pot does not mix at all with the vine! The Yerushalmi answers this question by saying "mi'Safek." The Pnei Moshe (in his second explanation) writes that this means that the hole-less plantpot does indeed render the wheat and vine mixture forbidden, but only through a doubt. When the Mishnah in Kil'ayim states that it does not make it forbidden, this means it does make it prohibited mid'Oraisa but it is forbideen mid'Rabanan.

This is a proof for the Gilyonei ha'Shas that it may be that the Mishnah states "it is not forbidden" even though it is forbidden because of a doubt. Similarly, when it is stated that a woman is Mekudeshes, this means that she is married without any doubt.

2) There are further proofs that if the Gemara states "she is not Mekudeshes" there may still be a doubt, and when the Torah states that a Mamzer may not marry, this does not include a doubtful Mamzer.

a) The Gilyonei ha'Shas also cites Kidushin 48a where the Chasan says, "Be married to me with this Shtar." The Chachamim say that if the paper of the document is worth a Perutah she is married, but if it is not worth a Perutah she is not married. Tosfos (DH Shamin) writes that if it is worth a Perutah she is fully married, while if it is not worth a Perutah she is not fully married, but there is a doubt concerning this marriage, and if she subsequently marries somebody else she requires a Get from the second husband. We learn from Tosfos that even though the Gemara states she is not Mekudeshes, there still may exist a doubt about the marriage, and all the Gemara is telling us is that it is not a Vadai marriage.

b) The other proof is from Kidushin 73a. The Torah states, "A Mamzer may not enter the congregation of Hash-m." The Gemara explains that it is only when he is definitely a Mamzer that he may not enter the Kahal, but if there exists a doubt that he may not be a Mamzer, he may enter.

I think this might all be useful for a person learning Maseches Demai, since this Masechta specializes in cases of doubt.

Kol Tuv,

Dovid Bloom