I can say for sure that Jace doesn’t show up automatically. In chapter 10, when Perry is breaking in, you have to choose the option to call someone for help and then call for Jace or he won’t show up. You do need to have a positive relationship with Jace. There’s a couple ways to do that. If you go to the doctor’s office, you can tell him to call a realtor. Offering the room in your house will also give you an increase in relationships with Jace. Also I believe if you ask Jace what his deal is if you get attacked by the McCraes, that’ll improve your relationship slightly. If you choose to follow Jace in chapter nine and you get caught, you’ll lose relationship points with him.
I thiiiink Jace can still show up after the fact if you rented the room out to him, though, if you didn’t call him. But if you had an extended scene with Amanda and Monroe fled the scene, then it wouldn’t happen. I’m pretty sure he’ll only show up if you reject Amanda completely, get her out the door, and then talk to Monroe. After Monroe leaves, then Jace should show up to check on you. That’s where you should get the romantic scene, as well as the epilogue. I think you also need to have sided with the thin bloods and won to get that epilogue.
I’m not sure about Monroe ending up dead. If you successfully defeated Chastain in the final chapter and Monroe survived, then there shouldn’t be any issues. Maybe a glitch?
Re: Becoming a Kindred: In the world that Out for Blood is set in, the World of Darkness, thin-bloods canonically cannot embrace humans. Only a full-blooded kindred can do that. That’s the reason you don’t get any offers from the thin-bloods. Only Amanda can give that offer, and it only happens if you side with her and end the conflict with the thin-bloods. Then, in the epilogue, she’ll offer to embrace you.