
http://www.mangago.zone/read-manga/kenji/mf/v19/c005/12/ So when did he actually learn Xinyiba? Did he steal the technique from the bandits?

Is there a oneshot or something which shows why Maya noticed Nemu in the first place?

Closest thing we have to something like that is a brief flashback in Imasara Briefs, which still doesn't really explain if Maya began to like Nemu /because/ he saw him in his briefs, or if that whole incident even happened because he already liked Nemu before that.
I'd really like to go more in depth about it too ;;
Great story, but the author clearly forgot the part where that poor lady stayed friends with the family, and was a bridesmaid at the wedding and had a happy, fulfilled life until she died. It's awful that she was just left to die off all alone overseas.