To get back to the original subject of this thread, I propose a 4th category for what might be called "temporizing" words; i.e., some sound, if not a real word, which allows the speaker to gather his wits. Some illustrations:
1. Major Hoople, in the immortal comic "our Boarding House", was fond of "Fap!"
2. In a Dorothy Sayers novel set in Scotland, there is a Scottish policeman who is fond of "Aye, imphm."
3. In a detective novel, I forget which one, there is a character whose use of "Mmmf" constitutes half his dialog.