But what I never hear anyone discuss is how the payments on the HELOC, whether interest only or principal and interest, factor the overall cashflow and return?

I can see this principal applying more broadly to AP subject you are raising, but also as the STR legal landscape develops (and the end game there seems to be eventually heading towards a more unified policy and even constitutional argument in front of the Supreme Court, in my opinion) - this may apply very readily to a sound legal strategy for STR protection? 

The builder makes monthly interest payments and repays the principal once the property is sold.

Becoming a Key Principal (KP) in other people’s commercial real estate deals is a great way to shelter your income, participate in several deal at the same time, minimize / spread out risk, capture equity w/ a preferred double-digit return at a discount cash-out-of-pocket investment. 

You can pull out the principal you’ve put in them over the years tax free, just not the earnings.

The drawback, ofcourse, would be that I wouldn't have much remaining in the Roth 401k.

The only adjustments I made were adding a couple additional rows of data at the very bottom of each page to track what I paid in principal each month, and what my net cash flow after that was.

Principal broker 20+ years in commercial real estate. 

If that is the case, we would have to discuss your offer and the remaining principal balance on the loan.

Syndications are EXTREMELY risky investments suitable only for highly-experienced accredited investors who can appreciate the risk both to their principal in the case of bad planning or a scam AND to having their capital locked up for 5 years+.