I have considered leveraging equity out of my existing properties to expand, but am not sure how it would work.

Seller finance deals work when one of the following situations is present1- property can’t be financed with institutional financing usually because property is vacant (commercial property) or in bad condition2- seller will only accept an above market price for the property and by offering seller financing opens the universe of potential buyers to include people who wouldn’t qualify for financing3- seller is able to obtain a “piece of the action” from a buyer rehabbing the property and hence ultimately obtain a higher price.4- the seller has an existing low interest rate mortgage and wants to “wrap” the existing mortgage into a seller financed higher rate mortgage and pocket the differential 5- property is an investment property, seller has a low basis in the property because of depreciation, and wants to spread out the taxes due.Are you thinking of trying to purchase with 100% seller financing, or are you able to /willing to offer a down payment of say 10-20% .  

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?