Not permitted - used by official Core MIDI documentation so I think it's better to stick with it.
Lengthed - I understand, super strange word, but... maybe you can give me a hint: how to say that something has a length? One word, adjective is preferred. Also this word unfortunately is pretty old within the library and thus renaming will cause breaking changes :(
As for SysExIncompleted/SysExCompleted, thank you, I'll fix them.
Fixed some spelling mistakes in comments/docs and internal code. Also consider changing these: