Let's say that I wanted to actually sell a house here and a house there on the side, for supplemental income.

It's going to be priced pretty high as the owners have gone all out to make it attractive to yuppies who want to live near DT with quick access to all Austin area hot spots, restaurants, coffee shops PLUS have income from 2 other units to supplement high mortgage payments. 

Supplement that with anything you know and it sounds like that's your best shot.

My goal is to build enough residual income off of real estate in order to supplement the $3,000 a month I'll get from my disability check every month.

Beyond that:Two great books I have read and would recommend are both by Dave Lindahl:Multifamily MillionsEmerging MarketsWhile I have not read the following books myself, other people I respect have recommended them:Steve Berges - The Complete Guide to Buying and Selling Apartment BuildingsCraig Haskell - How to take an Apartment Building from Money Pit to Money MakerThere's lots of great podcasts, too, which should supplement your book reading.

I kept the Roth so I could get  tax free money to supplement my taxable money.

In most cases it's a 4 or 5 unit or a property title that is not in your personal name;  LLC, trust, partnership, etc.If you have the duplex titled in your personal name, 99 out of 100 insurance companies will write a duplex as a homeowners policy, avoiding the need to supplement with a renters policy for you and your wife.so either your agent is new in the business or the insurance carrier has odd underwriting guidelines.Call a few local companies who write personal lines insurance and just ask how they would structure your policies.  

He bought a 3.5% 5 unit (50 k for 1800 rents) thought it would be a cash cow and would supplement his already laid back retirement..

Question 3:  Is all of the knowledge needed to  pursue any of the careers mentioned in #1, exclusively found in the classroom (paid for), or is it supplemented and enhanced continuously after you have "paid" you dues?

Use the 80% to cover rehab, and supplement with friends / family or hard money to finish the rehab.