If seller stays, write up a lease and treat her like any other tenant. 

I understand no advice here should be treated as legal advice, so it would be really helpful to tell me what you would do.Thanks BP community!

Treat this the same way you would any other type of new business, whether it be a grocery store, auto repair shop or landscaping business. 

That means that banks will pretty much ignore the LLC, especially if it is a single member LLC and treated as a disregarded entity, and treat everything as if it was in your name.  

A joint venture is treated as a general partnership. 

If you get a car for free are you going to treat it the same way as if you are making $700 payments every month?

Pays on time or early, is very neat and treats my property as if it's their own, is just financially responsible enough to never be late on rent but irresponsible enough to never save up enough money to buy their own house.

If you continue to go about the emotional route of treating your tenants, you're going to burn yourself out really fast.

You have one lease....treat it EXACTLY the same as if it was 1 person living there...no differentNeed to stay longer...pay the rent....One deposit is returned and they can figure it out.

For example, I could've spent hours researching what the exact legal (and practical) requirements are for vets treating horses.