So, I'm looking for guidance here, and also, if I am able to pull this off, how do I scale up to 6+ doors a year using the new construction BTR approach?  

There are 3 different approaches to exchanging funds, so you will want to develop a strategy around your risk appetite and financial goals.

Personally I think you'll have to approach this case by case.

When I approached the same major bank currently handling my mortgage, they told me I'd easily be approved for a 150k loan, cutting my monthly payment in half, and locking me in a 15 year rate of 2.8- 2.9% instead of my current terrible 5.5% rate.

That, combined with you saying you have poor credit and minimal cash on hand, makes your planned approach an uphill slog. 

What is the best approach to protecting both parties when partnering with an investor for a fix and flip. my thought was to purchase the property under my newly formed LLC.

It just adds a hurdle that with the right approach you can maneuver around it and prepare for it.

This was a better approach for me to limit my liability with only a handful of investment properties, probably different for a larger invenstor.

Naturally as I am not a realtor, I cannot estimate a properties value, but I would like to do (in some way) is estimate its relative value.With this approach I am thinking I can bring facts to the table when I find a place I like and want to negotiate on price (which I will leave for another post).

If you solely approach wholesaling with the mindset of, "What would someone pay me versus what can I pay for it?"