Should SCOTUS Chief Justice Roberts be impeached?

Unlikely. If Obama & Co were blackmailing Roberts, they wouldn't stop with just Obamacare. The smart thing to do if you were blackmailing someone in that situation would be only to use them on very very important things. Less likely that the plot would be exposed that way. But I don't think that Obama's team is that subtle. Typically they operate with the subtly of a sledgehammer, which means that if they were blackmailing Roberts, they would push him to vote their way nearly 100% of the time and he has not done that.