The things called “erva” in the passuk are absolute, otherwise there’s no reason for the Gemara to point to them.
It’s an asmachta. It didn’t say eyes are ervah even though the same place where it says hair and singing also says eyes, because in the Gemara’s place and time eyes were not gorem hirhur to stam bnei adam. Hair, legs, and singing were, so the Gemara made the point by making asmachtas. Besides, the Aruch Hashulchan (OC 75) says nowadays you can make a bracha in front of a woman’s exposed hair. Plenty of poskim go with this heter. They all clearly cannot learn like you.
The Ria”z is not saying pshat in the gemara per se, he’s saying what the halachic nafka minas are.
He doesn’t have to be saying pshat! If it’s takeh a nafka minah, it’s a good pshat, so I can ask why the Gemara didn’t use it as pshat.
there’s no explicit contract on das Yehudis
It’s understood. Hence the ??? ??? ?????? said during the kiddushin.