The results from the crawl by CP are answer my initial two questions very clearly:

The AJAX fails because the crawler doesn't know from JavaScript and doesn't attempt to retrieve the fragments.

The zoomable images fail because ditto.

Also, I notice that the crawler inserts its own JavaScript (/wb-static/js/ait-client-rewrite.js and //five.partner.archive-it.org/static/AIT_Analytics.js). We'll have to check that this doesn't actually break existing JS on the pages. It does appear to interfere in the operation of the browser's history object; this is not likely to be a problem for this project, but GN is in the middle of developing something which makes active use of recent ECMAScript functionality for manipulating history; this is definitely something to be aware of.

I'll have to go back to my rendering pipeline and make everything a standalone complete file, with all its references embedded. Might as well do that in the XML, prior to generating the HTML.