They were compromised. Greed overtook the principles on which the project was founded and put the project at risk. They agreed from the start that Micay would own the project and hold the keys. They explicitly accepted those terms. Despite this, they tried a hostile takeover anyway.

Forking and building a separate build isnt dual signing, its just forking. You can do that right now with GrapheneOS and its build guide if you want.

Im not sure what you mean by the last part, GrapheneOS has been quite upfront with all of this from the start.

From a security-minded user perspective it makes sense to destroy keys when instead of a single entity I receive updates from I get another entity that is not equivalent, and half of my previous entity thinks that the other half is sus.