Which sex are they, female and male, right? Check every week, and get a second opinion if possible.
"lying on", you mean humping or just like in the one picture below?
Right now aggression is not your biggest worry, most likely not going to happen if m/f, m/m could mean trouble a few weeks down the road, f/f would be my least worrxy and I wouldn't seperate as long there are no problems (does humping is normal and a way to settle things - which works most times, but not all characters match). Bucks humping is a different social context than when does hump. Not the same thing. But I had does get pregnant at around 15 weeks - and my rabbits are 9-10lbs, smaller breeds mature earlier. If he's already practicing what nature tells him to do I wouldn't wait long anymore with seperating them, especially when you don't know their exact age. Best would be to get him fixed right now, it's quite normal here to neuter males before they fully hit puberty, but for some reason that seems to not be done in the US?
Sure, they like each others company, rabbits are social animals. But it happens so easily to end up with way too much, little, furry company.
Right, potatoes aren't good food for pet rabbits, it always depends on the amount and situation if something and how much of it is good or not.
Yellowisch feet are just a cosmetical problem. If they aren't going to a show I wouldn't do anything about it. There is no point in stressing them out for nothing.