My own opinion is by rationale of why the 15 year rule exists (to not discourage new building) it should not apply to either building.  

It's like they never existed. 

The structure has to either already exist (we converted the garage) or you can do an addition to the property and build it out that way.One of my clients took an illegal garage conversion that was expanded into two bedrooms and used the FHA 203(k) loan to get it permitted.

Does such a service exist? 

The REIA exists to support new investors. 

Mid 1980 or newer you should be fine (they allowed existing aesbestos containing materials to be used up after the ban).

or a broker that specializes in private loans if such a line of work exists?

So all I would be doing is ADDING more support by filling in the gaps on the header with 2x4 studs, leaving the existing 6x6s in place, and closing in the walls. 

Where deed is transferred but existing financing stays in place.  

The deals are harder to find in my experience but they exist.