how can one buy more properties when the current ones aren’t sustaining themselves in the view of the 50% max DTI rule? 

The market has already proven that obscene pricing can not be sustained unless income ratios increase accordingly.Not only that, it will be a long time, if ever, that the mortgage industry and Wall Street will get away with the raping and pilaging they accomplished in 2004-2007.

I was able to cash flow it for about $300 a month but I paid for it w/ the amount of damages sustained by the tenants.

Spending $10,000 to learn is reasonable in my view as long as I can see a return as a result of the training, compared to the loss and disquiet you'll sustain if you can't sell the home you just put 30-50k into and possibly a hard money loan.

The sustained lower rate of 7%, from 8% just 45 days ago, did bring a tick up in mortgage applications, which increased 7.4% on a seasonally adjusted basis from one week earlier.

Population change.For rents to outpace inflation. the rate of population growth must be significant and sustained.

I do think it’s a good idea to try and do both as long as I can and then I’ll have a better idea of how sustainable it will be full time to support my family.

And don't get me wrong, this seller's definitely isn't sustainable, but the solutions needed right now (ie: more inventory) isn't a quick fix.

Is renting by the room even after I leave sustainable?

If an area or state or even the US population continues to grow, well people need places to live and vacation, so to the extent that there is good demographics in an area and the sustainability of the income of the area, this will help dictate values and growth, or stability or decline.