Beninguman:
Yes Tosafos is quoting the lashon of the Gemara in Sotah – in the Gemara in Sotah it means that of the two seemingly contradictory statements, one was specifically referring to cases of a zivug rishon while the other one was specifically referring to cases of a zivug sheini. When transposed into Moed Katan the equivalent would be that Rav Yehuda Amar Shmuel’s statement was specifically referring to cases of zivug rishon while Shmuel’s statement was specifically referring to cases of a zivug sheini. Now you, can allege that I am reading too much into the words. But I think you have to grant that the words definitely don’t support your reading MORE than mine; at best it is inconclusive, which although I personally disagree with, I am willing to grant it.
As to your fourth paragraph, in fact according to your interpretation, the two answers are the same thing. Both are saying that there is a way to beat your mazal. So Tosafos’s question would not be particularly compelling. According to me, the two answers have nothing to do with each other.
Your final point, I don’t understand. All Reish Lakish said was that a person’s spouse is lefi ma’asav. According to me, the Gemara’s answer is that this is only true by your second spouse – your first spouse is predetermined and is not necessarily lefi ma’asav. I don’t see how it follows from that that you can’t have a sotah from a first marriage. If anything it makes it more possible to have a sotah from a first marriage because she can be bad regardless of whether her husband is good or bad.