If you plan to get into mobile home investing, it's best to study up on the product itself and network with other contractors who specialize in working on mobile homes should you need help with the fix-up work. 

When I delivered the notification about this fee she told me that I would have to prove that this person was there for more than 10 days, and said that in fact this person was only there for 5 days, since this person has been living for more than a month.What can I do about that?

If you go the 203k route, you'd want to get a 203k consultant in there to do a feasibility study on what the property needs, and about what it costs.

When purchasing a real estate development property, what is the typical agreement as it pertains to an options period to conduct studies and determine if to proceed forward or not with the purchase?

The fact that you don't see or understand the impact yet doesn't prove your statement true. 

Can they prove how long it takes to rent a vacant property?

As for soft costs, you’re looking at a fair amount for a condo conversion-there are legal docs to have drawn up, reviews, surveys, recordings, setting up the HOA, reserve studies etc.

So in essence you are providing us with case studies!

I am exploring the world of creative financing and alternative financing and have stumbled upon the master lease agreement in my studies.

You have to prove your income through other means, like your filed tax returns.