Can anyone with an economics degree weigh in on the long/medium-term effects of all this deferment?

It is obvious that they are extremely overwhelmed as each landlord and each tenant will be effected In some way.  

How does Zillow manage all that data but more importantly which location is it more effective and correct in?

In the bread-and-butter price points in each market there shouldn't be as much long term effect, and it will quickly rebound.

If it is not directly effecting your living space then it will be there waiting for them when they get back.

Turning the 20k into 40k through my 401k isn’t realistic in 3 years - then again I’m not sure what the long term effect of losing out on the compounding interest would be.

You can report it to a collection agency but first send them a certified letter stating that this is a formal request and if they do not comply you will have a debt collection agency go after it and it will effect their credit and lending opportunities in the future - you will have to do it for all 3 because they're responsible for it collectively.

With the market as unstable as it is now and unemployment surging, the effects on the real estate market may not be felt for another few months.

Steven Covey's Habits of Highly Effective People 2.

We are hoping that the extreme social distancing will prove effective to slow the spread of this illness and that we’ll all soon be back to normal.