And all just so they can put an extra bullet point on their marketing. It'd be easier and less damaging just to make a standard extension.
On Windows and on most OSs, software is installed in a location that the standard user doesn't have write access to, and you require root/admin authorization to install. Chrome bypasses the OS security by installing itself in a way so that it can auto-install software/updates without the explicit consent of the user. If all software did this, it would soon be a security nightmare.
Very probably yes. One of the best ways to fend off viruses is ublock, after all.
Unfortunately, AV vendors have not really demonstrated the ability to do anything like that in a reliable or secure way.
If Chrome has a security hole that the AV vendors know about then they can fix it at source. This is a lot more robust than opening a giant hole in Chrome's sandbox in order to insert its own code inside.