That is an excellent question. Thomas may hate his demon, but does he comprehend who he will be without it? Gone are the good looks and the perfect skin. No more super strength or endurance. Thomas will not be able to help his brother in the same way that he is accustomed to. He has said that he wishes his demon gone, but has he lived to long with it to fully enjoy life without it?

If you go by the soul gaze Harry and Thomas had, there is some connection between Thomas and Harry. In the soul gaze their arms are connected with their fingers/claws digging into each others forearms. To me this suggests that there will be some loss in trying to separate them. The question is what and how much of Thomas will be removed with the parasite.

To summarize, based on my understanding of the timeline, I don't think it would have been easy for Harry and Carlos to reconnect after their last meeting in Changes. Once again, from Carlos' point of view, Harry was at the least a colleague if not some degree of friend. Harry killed the Red Court, died, went to work for Mab, and did two major events all without talking to the council or touching base with the wizards who were his colleagues. I can see how Carlos might think that Harry has gone over to the dark side or at least is no longer on the good side.

It is and it isn't out of character. The last we saw of Carlos before PT/BG was a brief scene in Changes where Carlos chases Harry down after he confronts the Duchess in front of the council. At that point Harry and Carlos were still presumably on good terms if not outright friends. It was a little jarring to go from that perspective with Carlos to PT where there is simmering distrust/hostility and BG where there is a definite break. That being said, since we saw Carlos, Harry (in the council's view) singlehandedly wiped out the Red Court, seemingly died, came back to like, is in service to Mab as the winter knight, helped the Denarians break into Marcone/Hades vault. In addition, he has not been fulfilling his role as a warden and regional commander and as far as we know has not talked to the council once since Changes. It is easy to see why the distrust of Harry has grown and how Carlos, in addition to having that distrust/paranoia bleed over onto him, might also be feeling slighted because Harry has not reached out to him at all during these major life events.

But that's not out of character.  Harry, as Carlos pointed out, hasn't been acting like a friend, for a long time.  From a Council POV, Harry looks suspect, and he seems uninterested in alleviating the suspicions.  I'm not sure Carlos still believed Harry was his friend, whether or not Carlos thought of himself as Harry's.

I suspect that people like Victor crop up all of the time and the White Council deals with them routinely. Victor was different because he had access to a very powerful killing curse, had a burgeoning drug empire, and had a fairly powerful demon that he barely controlled. I suspect that Nemesis propped him up in order to threaten Dresden and/or Morgan. Both would be positioned to be major threats to any play that Nemesis made against Demonreach so would be good people to eliminate as early as possible.

I can't be enthusiastic about the possibility of Butcher following GRRM's path in getting involved with TV/movie adaptations.  With 5 or so years between publishing Dresden books since 2015 and with, what, 7 more to go after Twelve Months is scheduled to be released in January 2026, I'd much, much wish that he focus on finishing Dresden.

Or he can also get a co-writer and start pumping Dresden books out every 12-24 months like he did through Skin Game. 
Even then, the Dresden files wouldn't likely be done for 10 more years.

Excellent point. I admit that I was thinking younger wizard as thrall not younger wizard as ally at the island. This wrinkle opens up a whole new world of possibilities. Does anyone know where Carlos is during this fight on the island?