How do you think the 2 relatives will feel about having their rent raised or being required to pay rent period?

Review this periodically - 3-4 times per year, and UPDATE it.

May not be your cup of tea but they are cost efficient and they work to add passive income at a fraction of the cost to build a SFH.There are a few "Tiny Home" builders in Tampa and Manatee doing a ton of business.  

And where I could boost the rents 20% over a four year period.  

Typically, you can only do one rollover per 12-month period (again, IRS Publication 969 has the details).In the end, having the option to invest your HSA funds is usually a better bet than letting them sit in a basic savings account.

They then missed out on several more years of growth because they got out of the game and waited on the sidelines for things to look better. 2010 to 2012 was the period that cash investors were the players or those with IRA keep in mind investor loans were still basically non existent in those years for all those expect very well to do Investors beginners were not getting investor loans. 

Would you share 3 specific things I should do during my "training period" (if it includes a book, which one)?

This isn't 2011-2013 days but many people are stuck in that time period STILL.As I finish my licensing study, I'm very grateful I chose this route as opposed to being a one trick pony.Wholesalers did themselves in by their method of operating.  

With retirement in five years, your focus should be on balancing cash flow, risk, and tax efficiency.

Use financing but streamline contingencies and shorten contingency periods.