She will be paying rent anyways with me so her paying me rent and building our future wealth when we do get married is a much bigger win than us both paying down someone else's mortgage. 

If you don't own the house yet, you really need to pay the guy otherwise it's a waste of his time.Sometimes roofers and pools companies will give an estimate based on google maps. 

Many lenders offer competitive introductory rates, so if the goal is to pay off ASAP you will benefit from this. 

Now you need to figure out how to find deals and pay for them.

I pay an accountant well to make sure I have as little profit as possible :)For a small operation Google Sheets could work. 

transacting - buying and selling - is expensive and can consume years of gains, with all of the commissions and fees you have to pay. 

Insurance would cover it, I would pay the deductible, and no assets would be lost.If you are in an area like San Diego where people are more likely to sue, a judge is more likely to find you guilty, and the payout is expected to be higher, you may consider an umbrella insurance policy.

The issue I run into, based on what I can put down, is that I would most likely have to split the mortgage down the middle (I pay half/Tenant pays half) based on current rental rates and current mortgage rates (considering what I can put down).

If that is the case, you sell the house, pay off the HELOC, and use the $15-$20,000 to get into a new house.  

DIY will be the "cheapest" - you can set up an account on the MA DOR website, register your LLC, you pay over the whopping $500 MA charges you to register an LLC, and voila, you are off to the races.