The Tosfos Yeshanim answers Toafos' kasha the we didn't yet know of Rebbe Shimon. The problem is that the Gemara mentions this Shita of Rebbe Shimon to ask a question. It wasn't introduced as a Terutz. It's hard to say that as of now we aren't aware of what we are about to quote in a question.
His second Terutz is likewise puzzling. In the first Braysa, if it were Rebbe Shimon there is no Safek and whether or not this is Rebbe Shimon we still have to say Rebbe Shimon didn't understand the Chachamim in the second Braysa.
Perhaps the Tosafos Yeshanim learns the the Meisivei of earlier is this same Braysa. First we tried evading this Terutz of Rebbe Shimon being unsure, but the Kasha was emphasised again until we had no choice.
Tosafos explains that if the Chachamim would be using the term Hagrala to mean Hanacha then Rebbe Shimon wouldn't use it to mean picking the Gorel. Only now that they might mean picking it can he use that term for the same thing.
My problem with this is: If the Chachamim truly mean Alah Hagorel then he would keep quiet. He is only raising his objection to the fact that they might mean Hanacha. So why would he argue against them with what can only be understood as agreeing to them?
The answer is that he is using their term to say, "I know you didn't really mean what you said literally but I do." However, if the term would not be ambiguous he would have no reason to talk that way and use a word differently than them.
In essence we can take this a step further and say that the Gemara doesn't mean the he was unsure of their intent. He knew that he is arguing. But the point is that their words are ambiguous; their words are a Safek even if we know what they hold. This is why Rebbe Shimon is using their words against them, saying that he holds of what they say in the real sense.
Perhaps another Terutz to Tosafos' Kasha is that the Gemara doesn't want it to be a Kasha on Rebbe Yochanan in the Lashon that they argue according to Rebbe Nechemia.
For this same reason we'd have to drop what was said as the next step -- that Rebbe Shimon knew that the Chachamim really hold that it is Me'akev.
