Microsoft
My advice would be, build a test lab that mirrors what you have today. Back it up (so you can try, try again). Then reverse all the steps you did to build it, then apply SP2, configure it as you want (Multi-tenany is not just GAL segmentation, read the whitepaper we just published to make sure you have got it coveed), and test, test and test some more. Then once you are happy, head over and repeat the steps on production, accepting you will take an outage.
Or just build something new and migrate all the mailboxes. Not nice either.
And make a promise to never ever make the same mistake again. If you can't unpick the mess, and the helpful person who published a bunch of unsupported steps doesn't provide the way to unpick it, then you are in a bad place.
I knew a guy who hacked around in AD once, and three days later he crashed his car. Let that be a lesson to all of us.