I made some useful progress with the search functionality:

• All types of searches were returning multiple results, cascading through the hierarchy, and where one node contained multiple hits, it was returned once for each hit. Both of these problems are now solved; any node containing a hit is returned only once, and any exact phrase search returns only the bottom-level node containing the hit.
• The KWIC display was limited to the preceding and following sibling on an exist:match hit. This is no longer the case; it now takes the complete text of the parent, and creates a truncated intro and outtro to the hit.
• I've had a preliminary shot at thumbnails for hits on image markup annotations. I'm able to correctly retrieve the image and the dimensions, but for some reason my attempt to display only the hit area for the annotation is failing (I'm seeing a portion which is the right size, but is just the top left of the image). Nearly there, though!