As for the second time, the tenant had a medical issue which caused her to fall behind. 

If one of your workers (including your friend) get injured on your jobsite you could be liable for their medical costs and disability.

I use the following right now:REIT: Vanguard Real Estate Index Fund (VGSLX)Bond fund: Vanguard Pennsylvania Long Term Tax Exempt Fund (VPAIS)Fund prices will fluctuate with the market. 

My agent says I could switch my primary occupancy to Providence, and my wife can keeps hers as our home in NY so we keep the tax exemption there.

Did you ever file a Homestead exemption or did you originally have an owner occupied loan ?

Generally speaking, in my friend's state a primary residence property is exempt from property taxes due to the owner being a disabled veteran.

There are certain exemptions if certain requirements are met. 

All of my insurance policies have medical related coverage for lawsuits.To answer the question, I have my tenants in small multifamilies handle lawn and snow care. 

The seller probably has a homestead exemption plus maybe a disabled exemption.

The main difference is that if you use leverage to make RE investment inside of an IRA will result in UBIT tax on Unrelated Debt Financed Income, however income from leveraged real estate in a Solo 401k will be exempt from this tax.You may want to explore this thread for in-depth discussion:https://www.biggerpockets.com/forums/51/topics/527877-self-directed-solo-401k-for-real-estate-investors-q-and-a