What you have to remember with this Velocity banking stuff is that it really isn't this golden goose brilliant idea that some people make it out to be. If it was, everyone would be doing it. It's also the reason why smart people can't understand what is so "brilliant" about it, because it isn't brilliant or that helpful. People advertise it as a way to pay off a mortgage in 7 years etc blah blah blah it all sounds good to the ill informed or financial illiterate, but in all honesty, you could actually do that with any mortgage you want, you just have to be disciplined. The Velocity banking is more of a way to force you to diligently pay something off quickly. Another trick they like to talk about is how much interest it saves you, they typically show you how much interest you would pay over 30 years on a mortgage and take that entire interest amount as a % of the loan or house value, which is not a true % interest you are paying lol. And of course if you pay off a house in 7 years, you "save" a lot in interest, but you can do the same by paying off a mortgage in 7 years conventionally (albeit you would still pay a little more interest). This Velocity banking is more like one of those credit card hacking people where they have a slightly better rewards package on their credit card, sure they make a few bucks more, but the effort and time it takes to do it is not worth it to some people. Yes the method is probably slightly better than just using a conventional way of paying, but it's not absurdly better or genius. This is why it's so hard to try and understand why it's so much better, it's just not. Trust me, I'm a CPA and work in corporate finance and I've sat through a demonstration on this.
You make a very good point about opportunity cost with the HELOC and it's a huge detractor from this method. Why would you seriously care to pay down a 4-5% mortgage quickly if you are an investor and can earn 8-20% on your money. I posed this same question to the person on stage in a 40 person presentation on Velocity method and the presenter gave some ridiculous response that didn't make any sense.
I'm not saying this is necessarily a bad idea or a scam, but rather not some crazy good thing. The benefit from a HELOC is that it allows you to front the payment for a month, which lowers the average amount you owe on your loan throughout the month, and thus the interest you pay is lower, whereas if you didn't have the HELOC, you would have to wait a month to see that decrease in interest. They then say to take that interest savings and put it against your mortgage again and keep doing that and you have it paid off in 7 years. Ok sure, why would I want to do that if my mortgage rate is 4.2% lol, why not take that savings and invest it. They target peoples emotions "pay off your mortgage in 7 years WOW!!!" when in reality it's probably not the smartest financial advice.