We had a debate whether or not it is a service foul if you are on your tip toes, but your heel is "on the line". Not touching the line but just hovering over the line when serving. The lady with the arrow would be serving and she would be standing where the circles are, but her heels would be on the line in the air. Foul or not? I really couldn't find this information anywhere. I'm guess it's not a foul since you are not touching the line.
The server's and receiver's partners (like the two women in that picture) can stand anywhere on court. It is not unusual at all to have partners stand in the middle of the court, with feet in both service courts. It's hard to prove a negative, but no part of the rules mentions anything about hovering over a line. It's always the contact that counts: So yes, if the contact occurs within the lines, the rest of the player (heel, arms, racket etc.) can be anywhere on their side of the court.
Another good example where hovering over the line is not forbidden and happen all the time (you can see it in this photo as well) is when receiver in the doubles leaning forward. His front foot is almost touching the line but his body and the racket are half way on the other side of the line. No one ever was faulted for that (I hope )