As the website author, you can see which extensions the client browser is running, no? Wouldn't that make it your responsibility to detect then disable your custom keybindings rather than the users's?
Plus in the case of your parent, their website implements vim keybindings, so basically all of them are going to conflict with this extension, and it's going to be pretty useless if they detect that situation and react by changing all of them to something non-standard.
No, not really. Not unless it specifies certain `web_accessible_resources` which you try to load - but that's a bit of a hack, needs to be done on an extension-by-extension basis, and won't even work for all extensions.
The initial way we checked for it was looking for document.body.matches('*[_vimium-has-onclick-listener]') but apparently that stopped working in 2018. We then used a variation of this code: https://github.com/hackape/Detect-Vimium/blob/master/detect-... which seems to work still.