Aruch Laner

This morning, we had two questions that were unanswered and the Aruch Laner asked and answered both of them. I don't own one so I only have shacharis to look at it so if there are any mistakes it's either because I didn't understand it correctly or because I forgot.
1. The Mishna (nidda 47a) says that simanim below are the main thing according to everyone (the only question is if simanim on top are proof that there are/were simanim below). Then there are some other statements and then the gemara (47b) says "tanya nami hachi" that once the simanim are there on the bottom then we don't care about what is on top. What is the gemara doing bringing a braisa to support the mishna? Tanya nami hachi is usually used to support an amoraic statement not a mishna!
The Aruch Laner says that the girsa must be "tanya" and the words "nami hachi" shouldn't be there because it doesn't make sense for the reasons stated and we're bringing this here to show that this is only shitas chachamim but R' Shimon ben Gamliel argues.
2. The gemara on 47b quotes Rshb"g who says that sometimes simanei tachton come first and sometimes elyon come first depending where the girls are from. Tosafos points out that he is arguing on the Rabbonon and R' Meir in our mishna who don't differentiate between where the girls are from. Then the gemara quotes others who talk about which breast gets bigger first. Who cares?
The Aruch Laner offers two (similar) answers but I'll just quote one of them here. The chachamim said that if the simanim are there on top then it's proof that there are/were simanim on the bottom. Now we're qualifying that statement. If the girl is from certain places and she only has the siman on top on one side then it's no proof that there are necessarily simanim on the bottom.