Many thanks. Your explanation does seem to point to the reasons behind the issues we are seeing.
I was about to report back that even after clearing out the theme and resinstalling the issues remain.
The finer points of theme construction are more than I need to occupy brain cells with. I'll pass it back to the developer in case he/she has insight and in the meantime I've found a different theme that might do the job albeit without quite as much sophistication.