“Provide real estate brokerage services means to engage in any of the following activities: (1) for consideration, providing any of the following services for another person: (i) selling, buying, exchanging, or leasing any real estate; or (ii) collecting rent for the use of any real estate; (2) for consideration, assisting another person to locate or obtain for purchase or lease any residential real estate; (3) engaging regularly in a business of dealing in real estate or leases or options on real estate; (4) engaging in a business the primary purpose of which is promoting the sale of real estate through a listing in a publication issued primarily for the promotion of real estate sales; (5) engaging in a business that subdivides land that is located in any state and sells the divided lots; or (6) for consideration, serving as a consultant regarding any activity set forth in items (1) through (5) of this subsection."  

For 2019 and future years, maintain a detailed log as wellSide note; If you have Commercial and Residential, you need to treat them as separate enterprises for this purpose.

Can you transfer assets from a 401k to a Solo401k after age 70 1/2 for the purpose of avoiding limitations of an IRA?

Obviously a deal like that would be hard to come by, but for the purpose of this exercise let's assume. 

But those 2 things you listed file the sale way for tax purposes. 

In my auto repair business, I sub contract out some work, and I have to report what I subcontract for 1099 reporting purposes and also for workman's comp where I get surcharged on the subcontracting.

You can make an option to show up in your cash-flow statement.Mortgage interest is an expense and should show up on the P&LEscrow account is an asset account and you are not entitled to the deduction for tax purposes until the bank uses the funds from the escrow to pay for the home owners insurance/taxes at which point then it will be a deduction and show up on your P&L

The APRs afterwards are 18 - 28%, so I wouldn't use them for that purpose much after that. 

I buy each house I buy/renovate/sell in a different LLC, solely for the purpose of the investment. 

Custodian is required with this setup, but special purpose LLC is setup and IRA makes investment into this LLC.