Nothing should be an "estimate."

You can use the website affordablehousing.com to see the estimated rent for that type of property and location, but it is just a rough estimate. 

If I use an FHA loan to buy a BRRRR and house hack let’s say I’m starting with $50K cash and I put 3.5% down let’s estimate $12K including closing costs.

You may need to get a home appraisal, and you'll be responsible for the cost"My question is : Should I have my own appraisal done by a reputable company that would be accepted by banks for appraisal estimates and then request Citizens to do their own appraisal for a request of PMI cancelation.

How much estimating have you done on the costs to furnish it as an STR?

For partial asset disposition (like replacing a roof), estimate the old roof's value using the cost of the new one and write it off so you're not depreciating two roofs.Amending your return might not be worth it for changing the depreciation.

We calculated the collateral value of each property, and told him we would be willing to do a release based on (1) a payoff of our estimate of that properties collateral value PLUS (2) a extra fee (profit) to us of $50,000.  

My CoC can only go DOWN since my rehab estimate is pretty solid. 

Understand what retails properties sell, average prices per sq/ft, types of properties investors are looking for, what rents are, what areas sell the best for investors, what to look for to get accurate rehab estimates, more properties you visit and see in-person the better, and most importantly, dont be lazy! 

Then the buyer was paying closing costs, which I estimated around $11,000.