a. You can adjust the pass heading of grid flights by using the standard iOS two finger rotate gesture.
b. The path is automatically adjusted to line up with the takeoff point, which in the absence of some other requirement, is usually the shortest path possible.
c. A two point line mission could easily be approximated using a grid mission. The Linear Flight planner is designed for more complicated linear flights. You are correct, we do not allow for single pass flight because it won't create usable photogrammetry data. That, and the aircraft has to come back anyway so why not take pictures?
d. Google Earth and KML files is a much more universal way of doing things. Typing in coordinates is not fun for anyone. Map Pilot is a tool for collecting data and we try to provide a reasonable toolset to allow different things to be done.