You are making the implicit assumption (2) that Google's "fair share" includes some fraction of the money they make outside the US.

I disagree - I believe taxes are payment for government services received. Google Ireland and the Guinness Brewing Company did not receive government services from the US, and hence their "fair share" (to the US) is precisely $0.

Incidentally, the shareholders of Google USA may not live in the US. And similarly, the shareholders of Guinness may be located in the US. If it turns out that some large fraction of Guinness shareholders live in the US, should Guinness start paying taxes in the US for it's Irish operations?

(Guinness does, of course, pay taxes in the US for profits made by it's US subsidiary.)