I think that I may have stumbled upon a solution, although I can't say that I understand why it works.
In an attempt to remove extraneous data from the debug log when specifying module "mpegts", I went to tab "Configuration" --> tab "DVB Inputs" --> tab "Muxes" --> column "EPG Scan" and set the mux in question to "Disable". I then went to tab "Networks" and clicked button "Force Scan". Once the scan was complete, the services associated with the mux appeared on tab "Services" just as they should and I was able to successfully map them to channels!
I tested this numerous times: after going to tab "Services" and deleting any successfully scanned services from previous experiments, going to tab "Muxes" and setting column "EPG Scan" to "Enable (auto)", and then initiating a scan, the mux's services are never added to tab "Services". When this procedure is repeated with "EPG Scan" set to "Disable", the mux's services always appear on tab "Services".
Once tab "Services" has been populated, I once again set "EPG Scan" to "Enable (auto)" to get EPG information - no ill effects so far.
Thanks to everyone for your assistance. If anyone would like me to experiment and post excerpts from a debug log etc., reply here and I will attempt to do so.