The drawback is you need to build trust and relationships upfront.Both have their place, but private money tends to offer more flexibility, while hard money is more structured and accessible if you lack personal connections.

and they charge per door and not sure if they even offer everything that I want. 

Is the portal that the landlord and tenants use an additional charge?

Additionally, as stated above again, the projects that I was taking on were purchase well below market value.

Your son is likely paying more than the property is worth, by definition, as the seller would almost certainly prefer a similar offer that was not subject to. 

I'm currently renovating my rear unit and it has the perfect layout to split into 2 units by adding a wall and an additional front door.

If I have a capital provider whom wants to be 100% passive and is ready to invest $100k with me (cash), and I have all the syndication structure and offering documents for this project... can’t I just find an accredited investor, present them the deal / get them on board, and go through them?

The views are breathtaking and offer some of the most dramatic combinations of beaches, dunes, rivers, lakes and mountains.

Making additional payments or taking a shorter term are just depositing more into that savings account (it's not like the property appreciates more by paying down the loan). 

Additionally, regarding the legal structure, an LLC seems to be the most suitable option for our situation.