Okay, so despite my questions above, I took a stab at your latest (2017-M04-01-1700) suggestion, on a freshly installed Whonix-KVM running on Fedora 25-KDE. The following error is returned:
setfacl: Option -m: Invalid argument near character 3
Does this error refer to "u:libvirt-qemu"?? The latter does not automatically appear as a Fedora user as it does in, say, Ubuntu, when KVM virtualization is installed.
Based on this fresh install, I should also note that your advice from 2017-M03-21-1811 appears to have had some positive result: the shared folder is now respecting the "read" permission from both the host and guest side; previously that was not even a guaranteed. And, it's allowing simultaneous write permission from either side, albeit, limiting the write ability on each file to the side (i.e., user) that created the file. Previously, write ability seemed to be strictly limited to which ever side accessed the shared folder first, within the context of a given session. While not ideal, this is definite progress.