In both cases, you can still include clauses that essentially say if the animal causes issues on or with the property or with other tenants, that it can result in termination of the lease. 

Do I need them to sign a new lease for June and the rest of the July or will a 39-Day notice be enough since I already have rent payments and the security deposit? 

Store this event in your mind and if she pulls any future shenanigans like this then don’t renew her lease 

My lease ends in March '19, and I'd like to determine the viability of the following "live-in flip" plan from experienced investors and lenders in DFW:1.

  $50k in my market will get you one SFR that you can buy, hold and lease out.It will be hard to buy into this market and make a lot of money today. 

While I don't plan on doing any airbnb, I did have a guy that leased a group of units from me so HE could Airbnb. 

I need to lease my current condo within one month!

We attend an REIA where another member has inquired about renting properties from us - he would sign a lease, pay market rent, etc but he would be using it to run exclusively as an AirBNB.

Maybe she sells to you to pay the Ex and you write a lease to own with her so she gets it back over time?

We signed a 2 year lease through September 2018.