Yes, Misters Microfiction would suggest that he does indeed possess the cognitive faculty to make a choice out of free will. Mister when looked at by Harry’s third eye is the only thing which stays the same, I think this is indicative that Mister is quite happy being Mister, neither a mortal cat, nor a Malk, at least for the moment. This has been picked up in fan theories from the first up to and including that Mister was in fact the Avatar of the White God.
We have a story featuring both Mister and Toot in the coming year, and hopefully that story will shine some light on Mister’s back story. The two working together when they should be mortal enemies, except Toot is now a vassal of Winter and part of the Queen’s Guard, and the Malk’s Have long been affiliated with Winter. The two would work together to further an aim of Winter and that might push Mister choice. Technically the Knights are also responsible for their Courts Changeling’s, we know this from Ronald Reul who cared for and housed Summer’s Changelings. Has Harry been inadvertently been doing the same thing with Mister?
Besides just imagine a scene where Harry goes into check on a sleeping Maggie, only to find her cuddling a full grown Malk. Harry would have kittens. Cat Sith terrified Harry more than anyone in Mab’s Court.
WOJ has it that Harry is in for a big betrayal, everyone figures Carlos, Steed or Eb, but Mister? Mister has been with Harry I think even from Restoration of Faith. To find out the cat who has been with him for nearly two decades was Leas plant and Mab’s spy would be a big blow. To do that Jim needs to have used Checkov’s gun, providing all the clues in plain sight from the start. Think of how Jim did the reveal of Bonny, it was all there from Lash’s demise until his subconciousnesses big reveal. That something continued on with the tune he composed and could play well, the headaches etc. It’s like Bob’s parents, things have been seeded in from the start of the series for a reveal much later on.
Also Jim relates the story of his cat Fenris, who he says came into his household under false pretences as a stray kitten, when in fact he was a stray small adult cat. Fenris dominates Jim’s Pitbull Brutus in exactly the same way Mister dominates Mouse, a case of life imitating art though as that pairing occurred later than their literary equivalents. I think the false pretences line has echoes of Misters situation.
Harry is obeying Mab now, largely because their aims are running parallel now and Harry can see that. The need for a spy in Harry’s household is done. So time for a reveal.
If Mister is indeed a Malk Changeling, after him and Mouse, Harry is going to be very careful if Maggie asks for a new pet.