Your first question, I would say would not be a possible solution really. There isn't anything wrong with killing a lone hog as long as you are legitimately racing. If you're in a loaded rocket hog and come across an enemy that is lone, it makes sense to kill them because they are getting laps too. As a gunner - that's your job to stop them.
If you're talking about having someone on foot and nading a lone hog... then yes it is preferable that the person on foot only kill a lone hog if they are planning to take the hog. Of course, there are situations that arise that make it okay to nade a lone person: such as the lone driver about to run you over - you can nade them in self defense.
If you think about it as well, if it was illegal to kill any lone hog - then everyone would just race solo. It would completely crush the team aspect and no one would want to pair up anymore. The strategy would be lost because everyone would drive a single hog to gain immunity from the opposing team.
Unlimited nades are also a personal preference that we have set for a reason. We have tried setting map rotations to limited nades before. However, it does not stay popular and it isn't the game-set that attracts most of our players. By allowing unlimited nades, you always have the opportunity to hijack a hog or to defend yourself if faced against a loaded enemy hog. With limited nades, you could use up all your nades trying to just hijack 1 hog. If you failed at that, you then have no other opportunities until you either get picked up by your team, or get run over. As a clan, and also many guests, we just didn't enjoy those options. The limited nades servers that we have had, just don't have the popularity that the others have. The majority of folks don't like that kind of rotation, even though it can be a nice change once in a while.
