Ben, I know the Gemara sounds like das Yehudis can’t be d’Oraisa. The reason I entertain another possibility is the loshon of Rashi on the Mishnah, that das Yehudis means it’s nahug “af al pi d’lo ksiva”. He could have simply written “af al pi d’eino min haTorah”.
I may sound like a broken record, but, again, R’ Moshe writes that the chiddush of sa’ar is that it’s like basar; obviously, some form of basar is d’Oraisa. The source might be a kal ‘chomer from sa’ar, or there might be a different source which you and I are not aware of.
Is sa’ar had nothing to do with men seeing, t wouldn’t be uttar in the house. The reason it’s muttar in the house id because men won’t see it. See, for example, Rashi s.v. “v’derech mavui”.
I thought you had conceded that sa’ar was ervah based.