I am happy to provide insight as I work extensively with foreign national investors in the US.

What percentage of sellers are still contributing in some way, do we have data or knowledge of this?

R&D is like split testing, trial and error is "I'll be damned to spend the time investing in knowledge so let's just hope we survive the guessing". 

They are stuck in 2014 when notes were so cheap and easier to make money: now pricing is tight and even just buying to resell is challenging.Note investing can be more passive than other aspects of real estate but still takes a lot of work as well as a lot of experience and knowledge as there are many moving parts that can go wrong 

The most knowledgable people on market rents are the prospective renters who are currently shopping around trying to find the best option for themselves to live.

-At the end of the day, if it were me and I was a wholesaler (as opposed to a broker/agent) in this scenario, I would say that "as a courtesy", you can try to help and see if you can sell it to someone else off market, but you should not be expected to do a due diligence for the buyer, that's their responsibility, and you said from the beginning that you are not knowledgeable on land. 

Flipped a house with a pretty extensive mold problem but all drywall and flooring was replaced.

Invest in my knowledge/education of real estate rental investment through reading, listening to podcasts, and talking/networking with other RE investors, etc.

This is often easier in lower tier markets where the appraisals are not indicative of how the property would sell in an arms length transaction.The other risk you run is over extension.

For the last 22 years I've been involved in real estate as a house flipper, with extensive experience in building and renovation contracting, owned a residential property management company, and now have a private money lending business (licensed NC re broker).