Thanks for the reply. Your suggestions are indeed very relevant. I had already double checked and verified all permissions and ownership info for associated files, scripts, and services -- especially considering the zap2xml upgrade and the recent OSMC upgrade to Debian 9.3.
What I discovered, eventually, is that the recent change to zap2xml 2018-01-09 version, channel id's, is the issue -- as it is not backwards compatible. After I passed the -9 option (added as part of the 2018-01-10 version), my guide data was imported/visible.
I had suspected it was a channel id incompatibility issue, and spent time investigating potential issues with cache folder and the tvheadend epgdb.v2 file. After giving the zap2xml version log a REALLY good read, I realized the source of my troubles ... ;)
So, to be concise, I believe the guide data was there all along, just not displayed, since the channels need to be edited to map to the current channel id format ... socat was not failing. This mirrors your comments in your first post/original reply to OP. (Again, if I'd paid more attention, I would have saved myself some time ... !)
When I can set aside the time, I suppose I will drop the -9 option and disable channel EPG mappings, import the current standard version of the channel id's, then remap the channels ...