Regarding the conda build:
PyFerret will use whichever PNG library is used by conda in building the package. The PNG library is linked in with -lpng (without any version number), so it uses whichever PNG library is provided. The unusual (and annoying) addition of the version number to the base name is something the creators of the PNG library do. (The libpng.dylib internally calls itself libpng16.16.dylib, so then the system wants a libpng16.*.dylib library instead of a libpng.*.dylib library; similarly with .so libraries on Linux systems.)
Regarding displayed windows in PyFerret on Mac OS X:
This release of PyFerret on OS X High Sierra will allow one displayed window (a system error message is printed when that window is displayed, but it works). If you try opening another window, or cancel the one window and try reopening it, the program will freeze/crash. It works fine with multiple no-display window.
PyFerret on OS X El Capitan works fine with multiple displayed (or no-display) windows.
Ferret on OS X High Sierra or El Capitan works fine with multiple displayed (or no-display) windows.
Karl