One witness in Sota

Hello Rabbi!
The Mishnah in Sota 6, page 1, discusses witnesses who arrived after the Sota had drunk, and the Rishonim were satisfied with the ruling of one witness who arrived. Ostensibly, the Toss on Atar implies that the ruling of two witnesses is equal to the ruling of one witness (the Mishnah claims that according to the Toss, two witnesses are required), while the Rambam explicitly states that two witnesses are required.
The Rabbi (Sotah 3:23) explains that according to the Rambam, in order for the testimony of one witness to be credible, there must be an act of deception and contradiction and a prohibition of deception and contradiction. I would appreciate it if the Rabbi could explain what exactly each of these two concepts means.
In addition, the Rabbi explains that in light of this, the Rambam requires two witnesses precisely because the irrigation was done anyway and now one witness is no longer useful, since all his power is to prevent the irrigation. However, why can’t it be said that the fact that there is one witness harms the act of irrigation? I don’t think that this is necessary, but his words imply that since irrigation was done, it is beneficial, and seemingly why not say that the irrigation itself is harmed? Ostensibly, it can be said that this is a disagreement between Toss and the Rambam, I would like to know what the Rabbi thinks. Thank you very much.