Technology like that would be incredibly valuable, which means it will be developed if it can be.

I agree! The technology to read the thoughts and intentions of the human brain has some positive applications. I am sure Stephen Hawking would have appreciated significant improvements in this space when he was still alive. The right to private thought does not revoke the right to subject oneself to mind reading technology. Research could still proceed, and positive, lawful application of the technology could still benefit those in need.

You might possibly convince decent democratic countries to not do it, but if you think China, Russia, etc, would hesitate to get hold of this power we don't live in the same universe.

I agree! The independent governance of countries has consequences, but I believe its advantages outweigh those of global governance. I would rather that there always be a tension between many democracies and dictatorships, than a single governing authority.