Missing Rest Api Endpoints?

I have few questions about the REST API
Q1 : the official REST APIs docs don't provide any documentation about how customers should log in via API, there is no login endpoint!
Q2 : there is no way I could modify the customer cart (remove an item from the cart or change item amount) from the REST API, I can GET and DELETE carts but can't PATCH them or POST them
Q3 : if we want to create a mobile app that use the website REST API, how should we do it if there is some missing endpoint that are critical for the e-commerce flow?