Ick. I hate doing two-finger taps as it is. I don't believe one should need anything other than a pointing device with one button to browse the web effectively. Needing to hack in ways to simulate extra buttons is bad design. Why not use a keyboard then?
Even if Chrome always opened external links in a new tab with a left click, you'd still want middleclick functionality to browse the web effectively; I constantly open multiple tabs from the same domain- for instance, to open a HN comments section without navigating away from the front page.
People shouldn't have to specify which links are external and which aren't, it should be inferred from the URL being accessed. In the rarer cases where the server is saying to open an external link in the current tab or an internal link in a new tab, that should be specified in the HTML. For the people that want specific global behavior. i.e. all new tabs to be opened in current tab, that should be a configuration option in the browser.
I think you're confusing links and tabs. I think you're trying to throw a lot of extra logic into the browser that doesn't belong there.