PAA,
The phrase ???? ??? ???? ????? ???? ??? doesn’t mean that say the case over there was referring to a second marriage, it means that Shmuel is worried about a ??? ???, i.e. your second, earned, match. Shmuel was referring to a ??? ??? in terms of the how it would be possible that “yekadmenu acheir.”
The language in Tosafos in Moed Katan is just quoting the language in the Gemara in Sotah, meaning in terms of whether or not you have to fear someone else coming and taking your shidduch, this is something you should not have to fear if it your zivug rishon (predestined match) but you do have to fear if it is your zivug sheini (non-predestined match).
I am maskim that the Maharetz Chayes (mis)understood the Gemara like you and therefore has a question on Tosafos. But Tosafos understood the Gemara like me and therefore the Maharetz Chayes’ question falls away.
Furthermore, according to your reading of the Gemara, Tosafos should have asked a reverse question: Why doesn’t the Gemara in Sotah answer like the Gemara in Moed Katan? The answer in Moed Katan, that maybe someone will take her away with tefillah (i.e. break the mazal), which doesn’t require placing limits on statements that were made stam.
Most importantly, your way of reading the Gemara in Sotah doesn’t answer the Gemara’s question. The Gemara is medayek that Kinui is a bad thing and quotes Reish Lakish who would start Sotah by saying that a person only gets a wife who is a Sotah if he is/was a bad person himself. The Gemara then asks from Rav and the memra of 40 day prior to yetziras havlad. If the Gemara’s answer means that a person only get’s a wife he deserves by a second marriage that would imply according to Reish Lakish there are no Sotah’s from first marriages!?! Kenit zain a za zach.