The circuit shown in post#1 does not have a differential imput. the balanced MIC input of the Fishman Mini may have one, but we don't get to see it. That had me confused for a bit.
I have seen differential mic inputs used in a effort to reduce HUM pickup, and that can be effective if it is done right, AND if the input connections are actually differential.

 

Assuming this is true, what advantage/disadvantage would this have compared to the simple buffer config @crutschow suggests in post #7?

That output stage provides gain, along with a Sallen-Key filter, which the buffer does not.

 

The circuit shown in post#1 does not have a differential imput. the balanced MIC input of the Fishman Mini may have one, but we don't get to see it. That had me confused for a bit.