No questions, just an answer. You're wrong.
Mike Doonesbury -- the character in the strip, not the strip itself -- was a campaigner for independent candidate John Anderson in 1980, when the GOP nominated Reagan and Carter was the incumbent. Trudeau promoted Anderson's platform in Doonesbury, but his campaign never caught fire. In what was the closest thing to an endorsement coming from Trudeau that year, a week's worth of strips about a (fictional) Roland Burton Hedley ABC News special called The Search For Reagan's Brain suggested that Carter was the superior candidate because he was younger than Reagan, and the former's brain had shrunk less over the years than Reagan's had.
No, you're wrong. Several years ago, probably around the 2000 election, Mike was asked by his daughter about his political affiliation. He said point blank that he was a Republican. No doubt about it.