Shower Thoughts

Realistically, the easiest solution is not to do anything shady, but just to throw money at the corporate instances until they're a better experience than the non-corporate ones. People will flock to the ones that are faster, have more features, or cool extras and abandon the independent instances.

From there, they probably will eventually do a variation of your idea though, I doubt they'll try to make them illegal, likely just defederate based on them slowing down the network, harboring spammers, or find one small group on them doing something unsavory and then start the propaganda machine you mentioned trying to brand all the users on non corporate instances as supporting whatever that group was doing. Even easier would be taking over the development cycle and just implement breaking changes that require items that independent instances just can't keep up with.