How much it will affect your DTI depends on your existing debt obligations, income, the mortgage of the primary home you want to buy, and the mortgage of the house hack property.I recommend using the 5% down conventional loan product to house hack and only use FHA if you have to since there are more restrictions involved.

For the bathroom vanity Home Depot has a much more modern looking one with sink that looks similar in size to your existing one for about $170.

If I was you I would contact Rob Bergeron here on BP and look for existing multi family units that are on the market.

If they are underreporting their income to the IRS, I cannot guarantee that the revenue actually exists. 

I haven't done that because , I don't want to make people think I advertise non-existing deals, but he says he did it and it worked for him.

On this question, because the interest rates are so low on the existing loans, as you state, I would not do a cash out refinance as that would increase your interest rate and payment reducing your parents cash flow they are living on.

I am not sure the barrier you feel exists is as great as you perceive, or possibly doesn't exist at all. 

Is he (or someone like him) necessary to your existence in the business?

These individuals have reason to try to sell you services as opposed to actually telling you about the condition of your existing septic.

I had a few moments after my bizz day .. so I searched for the title company in the settlement statement and they do not exist on line..