What are the plus and minus between Hi/Low mount front derailers? Is there more flexing with a Low mount?

I think that you're asking "top swing" vs. "bottom swing".

Functionally, I think that they're exact trade offs.

Other than clamp size, cable pull direction and chainring size I can't think of any issues you might encounter on a recumbent. I can tell you that on a diamond frame bike flared tubes or water bottle mounts frequently limit which will work on a given frame.

Whichever you choose, front derailleur set up and adjustment is much more touchy than rear derailleurs.

"Low mount" (aka bottom swing) front derailleurs came about to fit on mountain bikes where the rear suspension needed room to move. They also work best on smaller chainrings since the difference in diameter is small. "High mount" aka top swing front derailleurs motion matches larger chainrings better since it swings upward as it swings outward.

Bottom swing FD on my MTB:


Top swing FD on my Gold Rush:

You got the important parts right but the low mount should be called top swing and the conventional high mount is a bottom swing.

Holy crap. And I thought the granny ring on my LHT was small.