With this in mind, you need to sign up for a service that notifies you of a price drop (many realtors offer this) if they refuse your first offer.This duplex screams AirBnB potential! 

Those are screaming to be short-term beach rentals!

The unfortunate inconvenience is that since the bottom doorbell is lit up, visitors or delivery people for the 1st floor unit assume that is their doorbell (when it's actually mine).

There's always a hospital visit or wreck or death that causes them to not be able to pay. 

There is only so much I can do on Friday night when other tenants are texting me saying the upstairs guy is screaming, and the person they are living with can’t do much about it because they have temper.

I also have a clause (lifted right out of Bigger Pocket's The Book on Managing Rental Properties thank-you-very-much) about anyone staying longer than 14 days is someone who needs to apply and be added to the lease if they meet the requirements etc…My worry is that the company will pay the rent, have this one person on the lease and then will cycle 'visitors' through the property essentially as a short term hotel.  

They threw a black bag over his head, drove him into the desert, stuffed him kicking and screaming into a small box, dropped several large prairie rattlesnakes in there with him, locked him up with the snakes inside and left him there.

The kids scream, Karen runs, and I go to look for the broom, but by the time I had my weapon the mouse had hid somewhere in our master bedroom.

That being said, two people per room (parent/child) is very reasonable.As far as the girlfriend, you should have rules in the lease about visitors to limit how many nights she is spending there.

She has agreed on the letter to make the house show presentable by the end of the lease date and allow visitors in while she stays for the extended three weeks.