Sounds like you don't have a traditional 1st mortgage on your primary residence, you just have a 1st position HELOC.

With 2 kids and a third in the near future, we're looking to move again in the next 2-3 years.We have around $15k savings, $45k traditional IRA and $20k in other investments. 

It's what I use on STRs and traditional rentals too.

Traditionally, the courts give a lot more weight to what actually happened over what is written on paper.

@Evan Polaski No , I am not talking about “ traditional STR play”.

We ended up going the HELOC route on that property for a few reasons. 1) the interest at the time was only 5%, which although seemed high, didn't matter too much with our potential property showing a 18% ROI 2) we leveraged a second home loan with 10% down for that new property, so our HELOC withdrawal was relatively small compared to what would be needed for a traditional loan on a rental. 3) I dont think I will see the rates in the 2% again in my lifetime, so figured I should hold on to that "free money" instead of selling it. 

If they are able to rinse and repeat this type of strategy to leverage equity in a property to secure another, is it because they don't have a traditional mortgage loan on the properties they purchase?

.- Streamline Process: Bypass the bureaucratic hurdles of traditional lenders.By leveraging hard money loans, home equity lines of credit, partnerships, and private lenders, you can boost your ROI and take your real estate investing to the next level!

So, how do you invest to (1) take advantage of property price appreciation and/or protect against inflation (devaluation of “fiat” currency) and still protect against a crash?  

Everyone knows the 3% or 4% balance transfer fee, but compare the fee to the interest you will pay over the same period with a traditional loan.If you have $10k in debt at an 8% interest rate with a traditional loan versus $10k of credit card debt with a 4% fee and keep both for a year (making minimal payments), you will pay less interest with the credit card.