In an ideal situation, the refinance, at a minimum, will allow you to recover all of the cash you put into it, and still have good cash flow.

Our financial situation is significantly different than yours, though, and you need to do what is right for you.

Why would a standard lease not work in this situation? 

If someone has any ideas on how I should handle this, or have dealt with a similar situation, I would greatly appreciate it. 

whatever you stance on that, it has created a situation where we (americans) look down on factory built homes.Without the artificial input I believe we would have giant , efficient, factories in every city pumping out modular homes, that are green, affordable and accepted as "normal" and probably cost as much as a nice new truck.If you have ever built a new home, the insanity of "site built" becomes apparent as the waste of materials and man hours can be staggering.

I've contacted the management company and explained the situation, stating that we are seeking a licensed plumber and if it's ok to start the job because, most likely, it's a common area issue.

Is a W9 required in this situation? 

It is hard to give a story without knowing the situation and what you want to get out of it. 

I’m a new investor looking for my first deal, my current situation makes the most sense for a flip.

Contact your local housing authority to get clarification, as you could already of run into a legal situation by even calling a service animal a pet.