These features aren't available.
-Ability to cut recordings in-game (Split the previous and future footage into two replays)
For this, there is the replay editor, so while you can't split recordings while you are actually in game busy playing (and recording at the same time) you can edit them after the fact using the replay editor.
-Stop/start recording in-game
For this request, it's actually been requested quite a bit, but it won't work because of how the mod actually works. When you join a server and start recording, the mod gets crucial information at that point, so to stop the recording then restart it the mod would not be able to gather that crucial information again. Not sure if that makes much sense but basically this feature would be hard to implement.
About the second feature, the mod could always passively record the crucial data (e.g. chunk packets, player uuid's, etc) and then only start recording the rest of the packets (e.g. Player movement, chests opening, etc) when the player wants to.
You could also take the approach of just scraping all past unimportant data when the player hits the record key and shift the timestamps of all the past important packets to timestamp 0