However, I have found out that with a conventional mortgage loan you cannot have someone else put down the down payment (gifting the money) unless they are related to you or a spouse/fiance.  

I'm trying to find a way to invest with him as the live in owner but not leave myself on the hook since it would most likely have to be structured as a gift which is a no strings attached  

Congratulations, that's quite the engagement gift!

I also would subtract a 100 or so from the calculation as gift to yourself since you will live there.

So it's a nice current year gift that makes you feel good and put more dollars in your pocket. 

You can receive gifts to accomplish this. 

Gifting each parcel to the other has no benefit that I can think of.  

One side issue, what if mom needs a nursing home.....I've read elsewhere that the Medicare folks are smart enough to ask: "does mom own a home or other assets that she should use to pay for her nursing home (and exhaust those funds) before Medicare pays more.....and they're smart enough  to ask "has she gifted her home to the kids in the last 5 years?

@David Bellmont Yes, this is all possible.You'll need to obtain financing to purchase all the properties... and you'll need skin in the game or a gift of equity. 

She cannot just, gift, sell or transfer 20 properties she owns personally to her IRA.