Or possibly do it automatically: when a catalogue only has a single category make some changes to how things are displayed:
- The breadcrumbs would skip going back to the catalogue's index screen
- The add entry form would hide the category field

Probably this would halve the implementation time and be a bit cleaner.

Note that you can workaround breadcrumbs issues already using the breadcrumb XML feature, but it's highly complex.

I've done a quick tweak for this.

If there's only one category in a catalogue, the category selector is not displayed and it will default.

If you use a tree catalogue then it already will skip the index in the breadcrumbs.
So that works.
You can also use a tree with multiple categories, but enable "narrow in", and remove the category browsing from the template. This way you have invisible categorisation, which is nice.

This sounds great, thank you.