Anyone can modify their code in whatever way they want. What happens upstream is for the project maintainers to worry about.
As for what changes Bitcoin, the attempt to push through CTV should tell you all you need to know about how much they can do unilaterally.
110'ers didn't just try running an alternate client, which many had been previously, myself included. They purposely forked the code, with completely expected results following therefrom.
Nobody can tell you what you do with your node's policy, and you certainly don't get to say what Bitcoin's consensus rules are. Neither can Core, Saylor, the miners, or the nodes. Consensus is emergent.
The market ultimately decides which fork wins. As it should be.
