why can't there be a method for making sure that such trust cannot be abused? Is this a tractable problem at all?
Which browser are you using?
Alas, the presence of this kind of reproducible build system would bring needed clarity to the chaotic ad blocker market, and the lack of that clarity works in Google's favor as an advertising company, so sadly I doubt they'd do such a thing.
Here you have to trust both developer's code and Google's build system. Can you verify all of the developer's codes? And can you verify how privacy-trustworthy Google's build system is? At the other side, you have to trust developer's code and developer's build.
I didn't mean which one you "should trust more" at all in my comment above. Please read again. What I mean is the first sentence here.