Her idea at present is going to be to purchase the house herself (using traditional financing - high interest rate) and make the payment affordable for the family's rent to cover by arranging the sale well below market value and putting up a down payment of 30+ % of her own money, resulting in a PITI of ~1500.

They may not be able sell their home using traditional on market methods, or even afford to pay off their loan.

If you flip to rent you will go more durable on finishings than you would to sell so you want to decide on that, but when you are done, you can see where you are at for sale vs. rent and decide, you would just have to move to traditional loan to carry it if you rent.

I have attempted to utilize traditional lenders however they will not loan on an investment property.

The videos I watched said that you can finance the property but it’s not the same as a traditional mortgage it’s kind of reverse and with a SDIRA property you need to put down like 50-80% and finance the rest because they’re most concerned with a large cash flow being generated to grow your investment and also to be there in case there’s any major repairs because those also have to come out of the account, you just can’t pay for a new roof for it out of pocket it has to come out of the SDIRA account.

This would then allow me to create a solo roth 401k and contribute a substantial amount more to it than just a traditional roth IRA.

Traditional FHA loan finance 3.5% down How did you add value to the deal?

After rehabilitating the property, you can refinance into a DSCR loan if traditional financing isn't an option for you.

Those doing best with ADU addition appraisals have an active role (flippers, developers, etc) and are adding them in areas that have traditional MF.  

It was my home but I’m buying a new home and want to make the most from it without doing traditional rental