One thing I'd like to see (and this is definitely doable on macOS side) is that if you trash a .app, the OS would automatically revoke all permissions to it.
If this were done then even with the sneaky reinstalling, the user would be alerted by a system dialog requesting access to their webcam.
If you have a ktext kernel driver to implement generic access to video input subsystems (think v4l) then why exactly would the user be alerted if the developer didn’t add the feature?
Read: anyone can write a kernel driver for macOS, you are too trusting of your software vendors. Get a hardware switch
No they can't. With SIP you won't be able to install it if it is not signed (for kernel extensions only a very few developers have certificates) and in any case you will be alerted about it. Also there are plans to completely disallow making kernel extensions in release after Catalina (since they can now run in userspace, I imagine that userspace will not get access to pre-installed hardware)
If this were done then even with the sneaky reinstalling, the user would be alerted by a system dialog requesting access to their webcam.