The networking is just BP for now. Thanks to Rama for inspiration! ( [Video] Player-Controlled Replicating Physics Movement, Simulating Physics! - C++ - Epic Developer Community Forums! )
Just to be clear - this is totally server-authorative, but the client runs the full sim, has information on all players current actions, and pulls them (gently) towards where they should be - the only time this seems problematic is when ping is >~200 or if a player hits an object that is not net synced! (which in a real level, there should probably not be any)
Not sure if I demoed it in the video, but there is a “jump” button, which works correctly.
After trying using the default replication system, I must say, I understand and share in Ramas happiness at being able to interact well over a network!
There is still lots of room for improvement - but I think this will now mostly become a side task, while working on other things