You want to have "min standards" such as 3x income etc to help you find a qualified tenant  

However, I would really like it if a lender could use my brokerage account as my “skin-in-the-game”, vs putting 10-30% down on the deal in cash.

There are plenty of loans out there that don't require income but they'll likely still need to check your credit, bank statements, tax returns, etc. to qualify you. 

Put a lot of the little extras in there that don't cost much but that people appreciate for good reviews (a coffee maker with pods, an ironing board, a few snacks , a sixpack of local beer, etc).Also, I use Airbnb to rent out my basement apt to tourists and to find places to stay when I travel.  

I know SDIRA (or checkbook) and solo-401k are both popular, but if I qualify for solo-401k, is there a reason not to choose that over the SDIRA?

Either way, he had to makes sure with Merrill Lynch that he is qualified for the money he is withdrawing without penalty.

You can also try to find a "cash rich" Partner to help you qualify. 

Because it's an inherited tenant should should offer a completely new lease at renewal time and yes you can ask for a deposit then and if they don't have it perhaps they don't qualify as your tenant?

If he's still going to make the mortgage payments for the next 6 months until he buys her out and gives her enough money I'm assuming to be able to some how qualify for the refinance, or some other purchase, then I'd tell her to wait.The main thing is if she can't afford the mortgage now, how is she going to qualify in 6 months from now? 

I have purchased my first note, and it is about to be boarded.