Yes, they wanted more power because they wanted to save the Union and eventually abolish slavery. Southerners refused to support Lincoln because they correctly believed that this was his final goal, despite the prevarications he made for political purposes. In order to bring about real social change, it is necessary to take calculated risks. Putting Johnson on the ticket was a calculated risk, and as such risks sometimes do, it turned out for the worst.

America may be more than voters, but it is wildly misleading to say "America" performed an action that was performed by a few dozen men who did not realize its eventual outcome, and then was roundly rejected by millions of of voters and professional politicians. When Johnson tried to win the presidency on his own, he couldn't even get nominated by his own party, which gave him exactly four delegates, all from his home state. Ulysses Grant refused to ride in the carriage with him for Grant's inauguration, breaking almost a century of precedent.

I like the rest of the article by the way. There's no need to overstate your case on this one point.