And, If you can pre-sell and/or get financing in Mexico at a reasonable rate, you can give a (much) higher return to US investors than they'd get at home.

What are the factors other than the interest rate when choosing between a Heloc or a cash out refi?

Your financing method is important, as interest rates will significantly impact your cash flow. 

I don't think its wise to refi since I'm locked into a great rate.

What is the interest rate ?

You will see higher returns through appreciation as the city is growing at a much faster rate. 

From cap rates and cash-on-cash returns to net operating income (NOI) and internal rate of return (IRR), how do these numbers help you understand their impact on investment decisions?

Rate is 7% 2.

Houston is a little expensive, especially when combined with current rates but there are some BRRRR deals that can work with decent ARVs...just as a quick example, here's a ranked list of properties by BRRRR in Houston - there's a few that potentially make sense but not a huge amount of choice.

U want your parents to talk to their CPA FIRST.. if they go the development route they are now ( depending on the amount of units) creating inventory.. which will be taxed at ordinary income rates ( very HIGH)if they just sell they can 1031  and move on to something else.. many times when you run the math consider the risks of a partnership.. the sale of the land is far better than thinking your going to make some profit..