I think you're right that a plugin install is smarter than trying to get people to stop going to Google. I think you need to make it very clear why it's worth installing the plugin initially. Something concrete that we all wish Google did.

Reassuring people that the plugin is lightweight and won't compromise their security is also very important. The history recording might be great, but it sounds creepy.

I don't know if it's true, but a dark themed web site might not be a good idea when trust is important. A/B testing on install ratio?