Hmm, interesting. I don't want to downgrade to 9.1.1 so that wasn't an option for me. I would think the CIFS share shouldn't affect the plugin install script but maybe it ran into some permissions error.
Well, I solved my own problem in another manner. After I posted my findings above, I thought a bit more about this error and figured since it errors out so quickly during package installation, there must be some permissions error somewhere. So, what I did to fix it was stop and remove all jails. Next, I browsed to the storage view and deleted the jails ZFS dataset. Once that dataset was gone, I edited the jails path and saved it to the same location, re-ran the plex plugin installer and bam, it's working now. I've been playing around with the plex app for the last hour or so.
Here's a tip I figured out after some head scratching. If you run into a permissions error when plex tries to read the media files, make sure to change the permissions on the files recursively so that 'other' has read access. I enabled r+x on other just to be sure.