I've now completed single player with OpenJK. It crashed (segv) after what I assume to be the final cutscene. There was also a nasty bug causing animation not to start in some cutscenes. Fortunately I found a workaround: quit the game, from the console if nothing else works, then restart from checkpoint. There was always a checkpoint at the failing cutscenes, which on restart ran just fine. Basic gameplay seems quite stable. So yeah, technically single player completable but needs some fixing.