1) Very nice question! One could understand that when Rav says we follow Rov in Mamon, he is referring to both types of Rov, Ruba d'Leisa Kaman (which is the statistic of cattle sales that they are discussing) and presumably also Ruba d'Isa Kaman.

Regarding Shumel, however, I see that the Mesivta edition brings a few opinions regarding in which type of Rov he argues with Rav and maintains that we do not follow the Rov:

a. Shev Shmaitsa (2:15): Only Ruba d'Isa Kaman. Shmuel would agree in Ruba d'Leisa Kaman, since that has a Din Vadai.

b. The Terumas ha'Deshen (314) infers from the Rashbam (Bava Basra 93a, DH Dehu) the exact opposite, namely that Shmuel only argues in Leisa Kaman, but agrees in Isa Kaman.

c. The Terumas ha'Deshen infers from Tosfos (Kesuvos 15b, DH l'Hachzir) that Shmuel argues even regarding Ruba d'Isa Kaman, not like the Rashbam.

2) I noticed that you are raising questions that many great Mefarshim grapple with. Yasher Ko'ach! For example, I saw a comment of the Chidushei ha'Iluy me'Meitchet that could interest you regarding this discussion. He points out an apparent Stirah in the words of the Rashbam in the section of Bava Basra that we mentioned above. At first, the Rashbam explains that Shmuel agrees that we follow Rov in matters of Isur, for example , meat from one of ten stores, etc. That is a case of Ruba d'Isa Kaman. So the implication would be that in a case of Mamon analogous to this, i.e. Ruba d'Isa Kaman, Shmuel would say we do not follow Rov. However, later in the Sugya, when Shmuel is explaining how his view can fit even with Rav Acha, the Rashbam indicates that Shmuel argues only in the case of Leisa Kaman, but he agrees to Rav in a monetary case of Isa Kaman!

Best wishes,

Yishai Rasowsky