- Am I able to do a mortgage assumption from personal to the LLC to avoid the balance being called and effectively keep the interest rate?

Louis, but I do have experience BRRRRing in a rust belt city.I think the challenge has less to do with the rise in prices, and more about how you will supervise a BRRRR from out of state.  

The lender would have had an appraiser issue a opinion of market rate rent for the apartment on your duplex so that's a start on what the rent should be.

Pricing - most bookkeepers I know are transitioning to flat rate pricing that includes software costs, based on the complication/volume of your business(es).

Depending on your situation, PMI may be higher and you may have a slightly higher rate. 

It probably helps that I currently have my mortgages through this bank as well because it has had the most competitive interest rates each time I have gone for a loan.

Well you cant BRRRR at 7% interest rate in a market that pays 3% Cap Rate on rentals.  

@Scott AlvarezThere are 3 main parts to a seller financing deal: down payment, interest rate, and price (4th would be the term).

I contacted a title company to officially record the sale, but they are requiring 3 separate closings (and tons of extra fees) because there will be 3 liens/private mortgages recorded (one is the original mortgage that I want to reassign since the interest rate was very low; the second will be held by my son to my daughter for his portion of the equity in the home; and the third is the difference which will be between my daughter and my other LLC). 

You can always do a HELOC on you primary up to 90% LTV vs an investment property only up to 75% -80% where rates are also slightly steeper.