Reviewing through your description I see where you also asked this as a case in your customer group, for reference, here was the reply:
I have picked up this case and reviewed the abilities within the custom email templates. I found that there is not a way OOTB to target custom template per specific user or groups.
I did a search in Ideas for Jive, but it does not look like there have been other requests for this functionality. You can create an idea and ideas with a score over 50 get reviewed by our Product team for inclusion into a future release.