WMG / Spellsinger

Picket and Nickum were disowned by Mudge and Weegee between books, or became literally Ret-Gone

Despite Mudge himself was a complete asshole and was completely amoral, he had grown up. And despite Squill and Neena picked up a lot of his attitude and thieving skills and rebellious natures, they were loved by their parents...which means Picket and Nickum must have really really screwed up, to do something bad enough to be disowned and never spoken of again.

Or, they weren't any worse than Neena and Squill, but they snuck into Clothahump's house one day, messing with his stuff. One of them fooling with a spell book chanted a forbidden spell which caused a mist to envelop the two of them. They never existed.

Neena had a Sex change between books

Her name mysteriously being changed to Noctor, that name sounds incredibly masculine. Perhaps with approval of her family, she paid Clothahump or Jon-Tom money to magically change her gender (Or more likely the two owed Mudge's family a favour, and after all they put Mudge through during the first 6 books, they probably owed him quite a few). She was very tomboyish to begin with, and perhaps the incident in the Baron's mansion gave her a massive fear of being raped, and thought it would be less likely if she was a male.

Neena was actually listed as "Noctor" on the back of Son of Spellsinger. It looks more like it was a case of the character's name being changed somewhere in the writing process and the memo not making it to the publisher, then Foster not checking his work for Chorus Skating.