The technology is useless if you can pass an integrity check just by running as admin. The point is that Google has control over what the process is doing and knows if you're tampering with it. I guess nothing would stop you from making a device's that uses the hdcp osd support to draw black boxes over ads you find using accessibility information, but if you're able to modify the page through extensions or developer tools or memory manipulation, then you're able to make automated API calls, and preventing that is supposedly the whole point of this system.

The reason for using an external device to overlay data on the video signal is that there is a browser API for tracking occlusion. It's supposed to be used for things like disabling animations of elements that are not visible, but could be unethically used for things like making you pay extra to listen to videos if you don't have an extra display to put them on.

I don't know why you think secure computing doesn't relate to driver control. Drivers run with special privileges and can modify protected memory. This is why people write root kits, and detecting those root kits is one of the primary motivations behind secure computing.