Transfers to be made in January transfer window could be effective enough to change the fate of the Champions League winner maybe. I can't imagine what would happen in case of seeing Real Madrid's signing a deal with Mbappe. However even if this happens he wouldn't join the team right away of course. They would need to wait for him until the end of the season unless they buy his clause to bring him immediately.
However truthfully I don't expect Real Madrid to sign him in January because probably he would like to wait for the end of the season to make his decision on his future.

I expect PSG to be eliminated early again which would make things really difficult for convincing Mbappe to stay.
Top three candidates and favorites to win the Champions League has got to be Manchester City defending the Champions League Competition, Real Madrid who have won fourteen times and lastly Bayern Munich who are also hot favorites having a coach who has experience winning the Champions League Competition before with Chelsea.
Paris Saint Germans wouldn't want to lose the big man Kylian Mbappe too early even if he'll leave at the end of the season, they'll want to try again this season but I doubt he'll be sold out.
I don't think the transfer window can change nothing. It's football though and it's always unpredictable.
I certainly agree with you about the favorites to win the UEFA Champions League this season. Real Madrid are looking too good at the moment with Bellingham killing it for them in every match they play. They dominated their league and they were the group topper.
Bayern Munich on the other hand looks too good as well with Harry Kane doing a very impressive job. Manchester City is a bit out of the touch right now due to the injury of some of the key players and they are coming back. I don't think Bayern or city will buy another player this January who could change the result of Ucl. Real Madrid could be the one. Mbappe might join Real this January and as @Distraction said, it will have a massive impact on the result of ucl.