See the interlinear for 1 Cor 7:2. The original Greek words both translated "his/her own" have two different meanings. "His own" (eautou) is 3rd person singular masculine reflexive ("his own/himself's"), "her own" (idion) is an adjective often used with things not exclusively owned by the possessor (hometown, nation, religion, tribe).

"given that the contexts in which this saying is used are only monogamous"

But it's not a given. There's no reason to suspect that NT Jews were never polygynous and one polygynous family (likely Gentile) is reported in 1 Cor 5:1.

"Not really. The main thing about Pre-fall is that we were without sin and in perfect communion with God, which we certainly strive for. Living in a pre-fall state is exactly what God is bring us back to and we are longing for. The items you mention as problematic aren't really (but do deserve some discussion, however I'm not going to go into those)."

Well we don't have to go too deep, but you're not hoping to being back sibling marriage, are you?