Looks like nothing comes into play until next year but the articles I've read are not clear on when the new rules will be implemented. 

According to the IRS rules you are considered to be disqualified person to your IRA and as such are prohibited from providing any services to your IRA:https://www.irs.gov/retirement-plans/plan-particip...If you had a long term tenant you probably could get away without using third party property manager, however vacation rental will have high turnover, which will require significant involvement from you, and would be considered as you providing service to your IRA, which is prohibited. 

There no reason to wait any time or have any “whichever is lower” rule come into play.

You get zero collateral in the equities markets.Either way you invest, just be sure that you have 'risk management' rules set up to protect yourself. 

There are cancellation of debt rules but we won’t go there because I assume you won’t default on your loan.

Starting 10/29/18 the new rules take effect where you can do home possible conventional 5% down on 2-4 units while already owning other property.

-Are rents justifying the increase or is there a variable factor playing a significant role that I have yet to distinguish?

Did you ask your CPA what the rules are for qualifying as a real estate professional?

They're nice people, but I can't justify giving them a 50% discount on their rent any longer.

You now have near or absolute control of the association and can implement rules / regulations that improve the property / community.