I also had horrific PMs, particularly involved with a 48 unit apartment complex in Houston.  

It isn't having paid off properties that offers security, there are still taxes, insurance, maintenance and management involved. 

I am reaching out for your expertise regarding a situation involving a seller from who I purchased a property subto an FHA loan in March of last year.

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Our current tech stack is a mix of Buildium (very clunky), Breezeway (great, but limited without a PMS), Xero and Google Suite tools with lots of spreadsheets involved.

Is there some other way to verify his involvement? 

There are a few options(1)You can get connected with the larger wholesaler firms in the area (2) go through PCPAO site and skiptrace the buyers of similar properties as yours, (3) hunt down the agents involved in the sale of properties similar to yours or go to local REI meetings and pitch it to the investors there. 

I Am looking at getting involved in rental properties and debating on which option would be best.

Quote from @Scott Mac: Every investment anyone makes involves risk.

So back to the drawing board, but for myself the big takeaway is to continue on the course that I set out on and too continually learn and stay involved with goals that I set out on and to stay focused.