Mike, the pumps design will dictate what sort of coupler you can use.
As I see it in your failed moter thread the pumps top bearing is in common with the motors lower bearing and below that is a shaft seal, right ?
Further, the pumps bottom bearing is just a bush running wet in the water, right ?
Therefore IMO you need a rigid self centering coupling and one that can transfer any pump shaft axial stresses to the bottom motor bearing and also precisely control pump shaft end float.
IME they are a split clamping type sometimes with a drive pin and/or keyway.
Some examples of what I mean:
In multistage centificals I've worked on or replaced the wet end there's a forked bush that you apply below the coupling to set the pump frame to coupling measurement correctly for correct impeller clearances, hence the wide access to the shaft coupler behind the cover plates.
In doing this mod with a non-standard motor to pump coupling and motor mount you will need to check for the impeller end float clearances in order to replicate them for proper operation and good pump life.
Certainly not impossible with the right gear and access to a machine shop to true the motor mount and make the split coupling.
Good luck.