Matanita and Matanita is

It seems that this does not quite fit

This is the language of the Gemara there:
“Mar ki beina milta pishit li masbara ki meshkana matnitata parka la. Rav Sheshet ki beina milta minya pishit li masbara ki meshkana matnitata parka la. Rav Sheshet ki beina milta minya pishit li masbara ki meshkana matnitata parka la.
He said to him, beina milta minya daipishit lah keh matnitata (Rashi: Masbara ki meshkana matnitata parka la, ke taduk mitnitata ke tzu kuvatai)
Beina minya bishet lah keh matnitata bishet lah keh matnitata (Rashi: Masbara ki meshkana matnitata ke tzu kuvatai)
Beina bishet lah keh matnitata He said to him (Rambam to Riva): "There is no Tanya Tanya."
And why did he say "Read" (regarding sin) and if it was cooked in a copper vessel [and polished and washed with water], and even in a small vessel.

A. So it is clear that Rambam is observing the Mishna, of course. Only Rambam assumed that his explanations are also consistent with conditional sources that he does not know. I described the language "matnita" as a Mishna, but I was wrong, and apparently there in the Gemara this language also includes barayat (Tanya), and even in them Rambam is observing and canceling his opinion because of them.
B. And since Rambam is less knowledgeable in barayat (and not in Mishna, as I mistakenly wrote in the question), this is indeed what is written there. But that was not Ribi's direct claim. Ribi did not claim that Rambach answers from a source and therefore there is a greater chance of finding a contradictory conditional source, whereas Rav Sheshet answers from a source and therefore there is a lesser chance of finding a contradictory conditional source. Rather, Ribi claimed that in the event that a contradictory conditional source is found (to Rambach's explanation or to Rav Sheshet's other conditional source) there is a difference from his point of view. This means that Ribi's claim stands even if the chance of finding a contradictory source is the same both when Rav Sheshet answers and when Rambach answers.
C. You are suggesting that if Rav Sheshet answers from a conditional source and Ribi comes and finds a contradictory conditional source, then a harmonization is made that actually uses both sources. I don't think that's the interpretation, but that the contradiction is absolute and there is a dispute over the conditions, and one must choose or determine what the conclusion is. Because if it is possible to harmonize, then it will also be possible to reconcile the explanation of the Rambach. Or do you mean that even if it is possible to reconcile, they don't do it for explanations, but only for contradictory baraita? Well, then why? In any case, in the case illustrated in the Gemara there, did he boil a vessel loaded with salt and rinse it, it seems that the contradiction is absolute and there is no room for reconciliation.
D. I am left with your answer: "If there is no reconciliation for the contradiction, you still have two conflicting mishnayot and you have your explanation on which to rely." What does this mean? We are not talking about practical laws that the questioner fears transgressing prohibitions, and if he has a tanna to rely on, then he reassures himself that he was entitled to rely on that tanna, even if in truth the second tanna is right. Because there it is about cooking holy things in a vessel (but maybe we learn from this for other prohibitions?) and in Babylon at that time there were no holy things. And it is not my business to talk about prohibitions. Rather, it is simply about a Rabbi wanting to know what the halachic truth is. And that is why the question is - if the chance of finding a contradiction to Rav Sheshet's source is the same as the chance of finding a contradiction to Rambach's source, then in the end, asking Rav Sheshet and asking Rambach does not improve the Rabbi's chance of knowing the halachic truth.
I came back to focus the question on the last sentence in your answer.

It is not a question of trusting even though it is a mistake (which really only concerns practical halakhah) but of trusting that the interpretation is correct. Even if one is dealing with practical halakhah, there is still significance in supporting my interpretation from a baraita or mishnah.

Do you mean that if this is a baraita against a baraita, then each person has a chance of being correct (let's say) 50%, whereas a reason against a baraita is, let's say, 1%? So a rabbi says that with Rav Sheshet he accepts a "certain" at least 50%, and with Rambach he does not?

This is one possible model.
It is also possible that if there is a Tanna like you then the explanation is clearly correct but perhaps there are other considerations that need to be weighed.