I imagine, Kemmlerian Necromancy works on some sort of WWKD self initiative. So if you are compelled on a debt with Kemmlerian Necromancy, you are forced to inflict death and chaos, even if what you are destroying or who you are killing is important to you (either emotionally or for your plans).
In the case of leylines, it is probably a bit more abstract, but still doable, I think. As a natural phenomenon, a leyline is in a kind of balance, that probably comes from its surroundings. If you tap too deep into a leyline for power, the whole system gets out of balance, and because the wizard tapped into the leyline and connected with it, he will also be out of balance, until he does something to restore that balance. Maybe the tapped leyline suddenly lets plants wither in its surroundings, and the wizard would have to try and restore this in order to balance the leyline again. Maybe he has to do some sort of shamanistic ritual to slowly fuel up the leyline again, during which time he is not allowed to eat or drink or even sleep.
Or a different approach:
Harry didn't want to cross over into the nevernever at demonreach, because of the dark leyline there and what might be on the other side. Maybe there is an entity connected to every leyline, only they are not that obvious, but they still come to collect.
As for outsiders, who knows, what they want? Maybe they don't even try to compel any single debt, but wait till it piles up to the size of mount Rushmore and then they compel them all at once to push one of their minions inside you to kill you from inside and take over your body, so they can slowly get some of their people in place to finally open the outer gates. There is a reason, Outsider Magic is forbidden, even talking about it.