The Due on Sale clause could be enforced by the Note holder since Title transferred. 

In my state, if there's no written lease then the Tenant is on a verbal month-to-month which is still enforceable and I can still file for eviction due to non-payment of rent.I recommend you present her with a written lease and demand she sign it or vacate.

OP needs to take the steps necessary to enforce the original contract or this seller is closing and not looking back.

I don't believe a judge would enforce treble damages for one day but can't be certain.

DOS enforcement is pretty straight forward as it relates to the servicing of agency loans.  

If I may, this is kind of off of your subject, but if a person is going to purchase a property and decide to use it for an Airbnb or short-term rental, make sure that the subdivision you're purchasing allows it because there's a misconception that you can have an Airbnb wherever you choose rather has an HOA or not even if it is not zoned, it's not going to end well for the homeowner because it takes one person to say no that they were against the idea of an Airbnb or short term rental in that zoned area then you're out your money, so do your homework first look into Code Enforcement for the zoning and Land and development just a piece of advice 

He cites South Carolina Jurisprudence to note that “while homeowners' associations typically have the power to regulate the use of common areas, their regulations cannot prohibit a usage contrary to any restrictions creating easements or rights of use of property in owners.”In short, if you were to press this issue and take us to court, all federal and state precedent overwhelmingly indicates the court would find in our favor, and this enforcement action would prove an expensive waste of association money.All that to say, we politely request that you drop this unlawful enforcement action.

No way to enforce it.

Texas requires specific language in the agreement to be enforceable.

If they want me to maintain the property, I will, but then they void the option agreement and we convert to a purely landlord/tenant relationship and the option fee is nonrefundable.I don’t get calls for issues, I don’t have a property manager, I just enforce the agreement (in a nice way).