Them getting the 100% versus the 65% collateralization will help relieve our debt burden really fast. 

Doesn't that make your mortgage larger and then you're taking on more debt? 

The only issue I've been having (so far) is being denied mortgage loan due to income/debt ratio being too high.

Live at your girlfriend's family's house, and get a property that is going to work for you, not a debt trap.Let me know if you need a referral for your future purchase in that area.

You don't want to take on credit card debt to make this work.

etc..If you are looking to leverage debt and come in w/ little to no money down (VA), I'd recommend owner occupying / house hacking / whatever you want to call it and depending on if you're looking at an SFR or duplex an FHA or conventional loan product will most likely work best. 

Locking in as much debt as you can at these rates is going to be a BIG deal in 10 years. 

You wont be able to count any of those rents because they haven't shown up on 2 years worth of tax returns yet, so this could lead to a Debt to Income issue.

All you do with a HELOC is transfer debt from one loan to another.

Over the long term, the rate difference will be immaterial because you have locked in a very low interest rate and can always refinance later if rates dip lower enough to justify the cost.I encourage folks to look for ways to invest without debt (equity partners, lease options, etc), but if you're going to get a loan, go for long-term, fixed rate.