Owning a fractional interest greater or less than 50% is not the issue. 

Student loan debt is also carried at an interest rate at a fraction fo CC debt.

Now, while I’m living there I’m only paying a fraction of the mortgage cause I’m getting payments from the MIL which allowed me to save even more money for other investments.

I think it could benefit in getting the slight reduction, maybe a fraction of the savings shared between you and the seller. 

As for controlling the cash flow, I figured that with me being off site and only putting in about 12 hours a week average, if they have any value as a manager they should be able to at the very least out perform my fractional efforts.  

Class.There are lots of places to find similar information might take you more work but at a fraction of the cost.

The upside is: I will lean on his expertise and the team he already has in place (agent, contractor, property manager), take on less risk with my investment being only a fraction of the downpayment (5-10k) and minimize the pain of vacancy with 10 units compared to a single/multifamily unit.

Thanks for posting and I hope some of this helps.you hit the nail on the head you can drive an hour or two from SF proper and buy properties for a fraction of the cost of SF proper. and you can buy SFR's in and around Chicago in the 200k range as well plenty of them. 

In both markets this cash flow at around this time is approaching the historical S&P but if you have financed the RE, the return is based on only having invested a fraction of the entire value. 

The college I am considering is a fraction of that size and it is not known for its partying.