However I just received a very concerning letter from the Fire Marshall Office saying I need to provide a notarized letter within 5 days stating I will not have more than 12 occupants in the cabin until I change the Certificate of Occupancy which would be devastating to my bookings.  

The seller may need to be the one who talks to them or you may be able to talk to them directly if you provide them with a letter from the seller specifying the loan #, sellers SSN, etc.

I've done deals like this where I've had the seller hold a small second, subordinated to the banks 1st position mortgage, with payments to the seller deferred until balloon payment which I make coterminous with the bank's mortgage.4.) 

If it came from your parents, you may need a gift letter from them stating that it's gift and does not need to be repaid.

Write targeted letters yourself saying you own and live in a multi in the area and are looking to expand.

Quote from @Donna Johnson: Quote from @Judy Foster: we too have tried a small investment in their notes and we got the same letter yesterday. 

Please verify with zoning and ask them for a verification letter so you can have approval on hand.

Thank for that information as I am having the same problem but the family is still in the property and won't respond to the attorney that send them a lot of letter and phone call.

I would send a letter to all units of the building reminding them of the clause in the lease you cited and let it play out from there.

I’m considering having a lawyer send them a certified letter, though I know that will still come with a significant fee, which is still preferable to full service.