Hi all,
It turns out that the issue was a simple reading and comprehension issue. The DB query for zx_slideshow v3 in the zx_slideshow.php template file only looks for slide group 'home', and I had put in the same slide group for each slide as I had used in v2.1/2.2.
The dialogue for setting up slides does not fix the slide group to 'home', I guess that the idea is that one can clone this template file in other places and have different slide groups as needed in those template files.
The slideshow now works perfectly. Moral of the story: before thinking about Javascript and DOM issues, check if there is actually any data to work with.