David,
You seem a little too hestiant for this short sale. I would stop worrying about all these 'laws' that come up with you research how to do a SS. Like someone else said, literally thousands of these are done every day and 'flipping' your SS to a rehabber happens all the time too.
You've pretty much made it past the hard part. You've convinced the homeowner to submit all sorts of documentation to their bank and gotten a bank to accept a SS. Now it's simple.
Sign Contract with Seller
Sign Contract with Rehabber.
Call title companies and say, "I need to do a double closing, can you do that?"
If they hesitate at all, call the next one.
Now give everything to your title company and they will start calling the banks and getting things lined up, make sure they know it's a short sale and the payoff figures that are being sent to them are your negotiated short sale amounts.
Why do you need to worry about the HML? That's the rehabber's guy, right? If he's a professional and has done this before he probably has a reliable financing source.
Don't get caught up in the legality mumbo jumbo. I swear before I did a SUB2 deal I was lying awake at night worried that the loan was going to get called DUE. I have never had a loan called DUE and have never heard of anyone having a loan called DUE either.