We're principals and wish to have our own money invested in projects that we sponsor.  

Those absolutely exist, don't get me wrong, but unless you know what the regulations are (at least in principal) you can't double check their work on your behalf. 

Some layouts take a percentage of the principal loan raise (2-6% in the case of GroundFloor).

Pay down the principal over the years that you intend to live in it so that when you leave it, the PMI will have been removed anyway.  

If life happens and your intent had to change FHA does not go against your need to sell.Fannie Mae Homestyle Loan (Great for investment property) One- to four-unit principal residences , one-unit second homes, or one-unit investment properties, including units in condos, co-ops, and PUDs.  MH is eligible, with the eligible renovation funds capped at the lesser of $50,000 or 50% of the “as completed” appraised value.

Will her monthly payments be going towards all of the principal?

Balloons also common at the end unless you pay additional principal each month.

There is one CA for a principal only and one CA for a broker.  

I just pay them their interest monthly and then give them back the principal once the tenant/buyer closes on the house.

On those properties you don't really care about principal pay down as much.What is important on these types of loans is no prepayment penalty.