I would focus on getting the consumer MVP done. Sounds like you already have conducted some interviews for the other two segments. If that is the case, then focus on developing a story highlighting the pain points and benefits for the payers and health system based on the interviews you have cond...
I think push vs in-app are two different things, like email and telephone call. Push reaches people who are not using your app, but require OS-level permission. In-app gives you much more flexibility but they only reach people who are using your app (obviously!). As to whether in-house or extern...
This is an article I came across a few years ago regarding the decision of adding social login: http://blog.loginradius.com/2014/02/7-considerations-using-social-login-on-website-app/ It does a good job of highlighting the key consideration points.