One lot has a 2 bed and one bath cottage that was an existing STR. 

For now, it Looks like your roommate/tenant can cover half of your mortgage (plus hoa fees). 

Instead of paying down someone else’s mortgage, you’re setting yourself up for bigger and better deals down the road.Your first deal is going to depend on a lot of factors—how important is location?

Any other expenses are using up funds that would otherwise be sitting in your checking account to make a mortgage payment with.

Going forward it would be good to specify that your property management software is the only acceptable method of payment in your lease (if not done so already).It might also be a good idea to check the pre-existing lease to see if they have any clause like that (sounds like you already did that).

A few months ago my fiancé and I bought our primary residence and we did really like our mortgage broker.

I've done a couple of direct loans, but for the most part, I work with a mortgage broker who brings deals to me.

The first question is, can I use a second home mortgage (10% down and lower interest rate) for this. 

You have acquired skills that are worth 500k, so maybe you can translate that into an adjacent career path and leverage your existing skills?

He calls and tells me "the income to mortgage isn't 1:1 and he is going to have to let the lender know."