Just like Communism and Libertarianism work until you add people, I don't think your simple workaround has a chance in hell of working once it's actually exposed to irrational people. Here's the thing I realized nearly a decade ago, no lesbian bridezilla is ever going to be content with a civil union. Her childhood fantasies are about the perfect wedding, not a "civil union". And if you try to tell her she's not allowed to have her marriage, she will gut you like a fish before you can get the words "civil union" out of your mouth.
The emotional attachment is to the word marriage. Changing the name won't satisfy anyone who's already up in arms about not being allowed to marry (they'll just see it as continued discrimination). Worse, your plan will upset a lot heterosexual formerly married couples, and would give the populists something really big to rant about. They'd be all over the blogosphere, the TV, the radio and the newspaper screaming mad over how you (personally) stole the marriages of millions of decent Americans. You could become the most hated man in America.
Additionally step 4 is counter-productive. Religious ceremonies should have no impact on the state. Instead during or after the ceremony the couple signs the paperwork for their civil union and it gets sent to city hall for registration. Which, I think, is exactly what happens now.
So what you've done is incurred great cost in changing the words used, pissed off a lot of people who are going to vote to rescind your changes and you've accomplished nothing at all other than creating a lot of very angry people who hate you (specifically) and you've guaranteed that your political party won't be elected again for a generation. Is it really worth all that to try and appease a group who, if you plan actually succeeded, would simply start complaing that gays should be allowed to have civil unions?