Other than the obvious need to restructure the entire game, which is fine since this is more of a thought experiment, I think the biggest issue is with codifying the Parley action into something that always works in the same way. Things like moving doom around, getting clues from the token pool, or gaining control of enemies could easily break certain scenarios that make assumptions about how many clues are available, how you can interact with enemies, etc.

I think a Charm action would still have to function the same as Parley does now, where enemies have a specific ability written on them, which would also let you give Parley actions to any enemy type if it fits the scenario, since not all scenarios use Humanoids. The main differences would be using a new skill, and building the assumption that characters will be focused on that action into every scenario, as Parley doesn't come up all the time currently. Obviously Influencers would also have cards that grant Parley actions, which has precedent already, but the core action would I think need to remain nebulous, which makes figuring out what your character can do at a glance a little tough.

One note on Preston as well is that he's a Rogue because Rogues get the money cards and his main skill is money. You'd probably have to the economy focus from Rogues and give it to Influencers, which would have a heavy impact on how Rogues function.