We also own some multifamily ourselves, so we're both owners/operators and 3rd party property management for other owners

Ward, et al. which changes the licensing requirements for parties who are the assignee of a home equity line of credit (“HELOC”).Crux is the state is saying Mortgage Lender License Required for Installment Loans Where Installment Loan is Secured by Residential Property even if you did not originate the loan.

Is % based on how much each party contributes?

If you are showing property only to one interested party at a time, I've found it best to tell them to call 30 minutes or an hour ahead of the showing to confirm they intend to show up.

As for selling with 10% down at 8% interest, you now have two title transfers that violate the due on sale clause and add complications to subject to “safeguards” that reduce risk if note is accelerated, if party in possession doesn’t pay taxes or insurance, and of triggering whatever system the lender has set up to notify them when a deed transfer takes place. 

People that want to rent need to work if they want to rent from me, for the most part. I

.- As with any RE transaction, it's best to consider the other parties perspective when determining feasibility.

My attorney told me because we are a third party there is nothing we can do and that I am at the mercy of seller’s attorney and TC/land surveyors.

Once you get everything set up, such as message workflows, guidebooks, pricing engine, etc, the questions and misc are not that time-consuming.The hard part is getting a good cleaning team/person and a reliable handyman.  

Even if your error was one that made the lease ambiguous, most states have laws that favor the party that did NOT write the contract (lease).Since your error was not even ambiguous, I don't see how you could fight it.