If you have a ton of experience and there is not too much risk on the money and a very good chance of making a large profit, then giving you a larger share of the profits might make sense if it's a better return than he could get in the stock market.

I would not invest in SFHs, too expensive to maintain, too much work dealing with one tenant per property and tenants devalue a SFHs.

Your expenses on maintaining a old MH will eat up any money you make over the term.

Your "empire" can be much larger than his and you have the choice of who maintains it. 

Whenever we purchase a property we must hold it maintain but it's difficult to handle everything together like organization or job, household, particular works alongside managing hire home so for that people employ home manager or property administration company.

You will have to take continuing education classes to maintain you license, but you can find most of them for free.Those are the minimum fees in order to get access to the MLS

The actual money may be maintained in the same reserves account, but, the way they are accounted for once spent are different.Maintenance and repairs are part of your ongoing operating expenses and are items that are relatively small in costs and routine repairs.

I like where we live now and we have a nice arrangement with my parents but I don't know if I should relinquish our ownership/stake in the property and cash-out and just rent from them or if we should maintain the current arrangement.

@Lucas LeBlancTo learn more about the solo 401k loan rules, see the following:https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-loansAlso, following are the similarities and differences between the solo 401k and the self-directed IRA.The Self-Directed IRA and Solo 401k SimilaritiesBoth were created by congress for individuals to save for retirement;Both may be invested in alternative investments such as real estate, precious metals tax liens, promissory notes, private company shares, and stocks and mutual funds, to name a few;Both allow for Roth contributions;Both are subject to prohibited transaction rules;Both are subject to federal taxes at time of distribution;Both allow for checkbook control for placing alternative investments;Both may be invested in annuities;Both are protected from creditors;Both allow for nondeductible contributions;Both are prohibited from investing in assets listed under I.R.C. 408(m); andNeither may be invested in your own business. 

I'm not sure where to even start to look at these multi-family homes and the process afterwards and maintaining it once done.