I've always held that one can make a principled, well-reasoned Libertarian argument both for OR against abortion, based on the grounds you just cited. But I've also personally always held that there is no objective, universal standard for deciding exactly when to consider that unborn entity a "child" which has rights and against which it makes sense to speak of violence being conducted. That being the case, I don't find any value in adding the specter of State violence to the mix. I'd rather leave this whole situation to the mother, her family, medical professionals and others supporters.

This is why I'm a Libertarian who largely opposes abortion on a purely personal level, but is 100% pro-choice on a political level. But at the same time, I can respect those who hold the other position based on the NAP. I'd just argue that with no objective way to resolve the dispute, it's best to not bring the State into it.