I'm looking toward retiring from my 9 to 5 job in about 10 years, and want to supplement my 401(k)/IRA and Social Security (if it's still around) income. 

More specifically, I am looking for investment vehicles other than "playing the stocks" in which to generate long-term wealth and an additional supplement to retirement.

Audrey your aunt would not want you in danger for monetary gain.Find another way to help supplement the payment if your aunt needs it.How many more years of college do you have before graduating??

The cynic in me says that REI is not giving them enough cash flow and they need something else to supplement it...

It isn't my best lead source by far, but it can be a good supplement.

My wife and I started out buying a two-unit apartment house about 7 years ago with a similar goal... buying 1-2 small multifamilies per year to hold and pay off the mortgages to cash flow supplemental income.

No I wouldnt I dont over pay knowingly ever but for someone who wants to supplement their retirement with out having to deal with houses all over the place over paying for the house next door might just work for them.

However, they are re-doing the exterior and have not collected enough or banked enough HOA fees to cover this exterior cap expense so there is a supplemental charge to each owner that was approved by the HOA. 

Should we put down a 20% deposit for our first home, or should we put down a smaller deposit and use the leftover cash to pay for an ADU that will generate supplemental income?

Acquisition fees and asset mgt fees keep the engine running from the beginning thru the hold period, the real money is made by the GP at sale or some portion at refinance / supplemental loan when property renovations are complete / value optimized.