I added new functions to AP_Mission that allow adding/removing a waypoint while a mission has already started. Using the scheduler I already tested them to some degree and they work fine. But it would be nice to use MAVLink, e.g. to add a new waypoint to the running mission. I found the guide provided at
Question: I guess I have to add something like MISSION_ITEM_INT? But in my case the field name=“seq”>Waypoint ID (sequence number) can be any value desired (except 0), pointing to my new add function (add_cmd(index, cmd))?

One more question. I have an SITL setup running. How can I send MAVLink messages to the simulation using C++?