Yes, Liz is the only NTR route, but you can take her romance one for your "Canon" run.
Killing Jake is not important, it only gives you two interesting scenes with him if he's alive, and one for if you kill him. I always keep him alive for mine, but I have to admit that getting to meet his Monster Parent is way too cool, so choose what you want to do.
Weird World has said that he really intended for a romance route with Mia, but, he shoot himself in the foot by making her look so young, since Patreon or some players could think she's underage or something. He'll change her design in 1.0 to finally make a romance route for her, though he hasn't said anything about cucking Jake.