I've worked with tenants in a similar fashion and ended up feeling ok about it but it was short term, over the course of a few months and they paid me something the entire time. 

Many banks wont touch deals that are sub 90% occupancy, so you could negotiate short term seller financing to help give you runway to make the building bankable for a refinance while you reposition it.

The bullet in the response that is standing out to me is:"(7) The accessory dwelling shall not be a short-term residential rental, as defined by City Code section 3-2-152(a)(2), for a period exceeding 120 days per calendar year."

I'm looking to build a Casita/poolhouse for use as a short term rental in Summerlin.

The short answer is No, you don't need it.

If I did short/mid term rental I could do about $3.5K/ month for 10/12 months.

A number years ago, I went to town buying a bunch of rentals in a short period of time and I can assure you they took money from my family instead of putting money in my family pocket.

The short answer is No, you don't need it.  

Miami is a great place to invest both for short and long term rentals and with condos, property management is simplified but be sure to ask about HOA fees as early as possible especially if you intend to go with a DSCR loan.

There is risk with this approach that the tenant can have short residence or leave in Nov to January.