It has drastically reduced my housing costs, allowing me to quickly save for my next house hack in 12-18 months.

I value that monthly rent more than the reducing my loan amount.

Even in the relatively few instances where you could partner with your retirement funds, the reduced flexibility and increased risk of a PT may not be worth it.

For the duplex I plan on renting out 1/2 of it and earn cash flow from the rental.

Now I know that not all investors are flipping, he or she might be buying and holding, so it wouldn't work in that case. in that cast, there is also such a thing as goodwill, meaning you provide the investor the deal without earning a commission on it, but you have most likely created a positive impression on that investor and when he or she does want to sell they would think of you first.

I’m wondering the model these agents are trying to emulate and charge for... in what way are they adding value to earn that extra 4.5%?

So if paying a heloc off on your sale reduces the net proceeds that simply means that you'll either have to take out more debt or add cash of your own to meet the purchase requirement.Whats more of a concern would be how and when you access the heloc. 

Appreciation typically reduces ROE unless the rents rise as fast as prices, which is rare.

The only thing you can do to reduce the number of useless leads is to include how much down and how much per month on the sign/ad/website.

If you think 'ill just put it on my personal card to get points' or the like you now need to earn $450, pay your 35% tax rate and have $300 left over to pay for it.