Just reviving this to describe my solution for anyone who might come across this thread because they have the same issue.
First, I’ve done a lot more with Ditto on other sites and realized that part of my trouble might have been a misconception about PHx being packaged in with Ditto (that what was built-in might be enough to handle modifiers such as the ones called here).
Unfortunately that didn’t have any change!
Luckily I also learned about Cacheable pages and PHx, so I verified that I had Cache-ing turned off on ~17~, which I did. Then I went through the bits and bobs suggested until it finally worked with the following code:
Ditto Call in ~17~:
[[Ditto? &parents=`2` &orderBy=`createdon DESC` &display=`1` &total=`1` &tpl=`gallery`]]
Chunk "gallery":
[[MaxiGallery? &view_gallery=`[+id+]` &display=`pictureview` &pictureview_start_pos=`[[MaxiGallery? &view_gallery=`[+id+]` &galleryOuterTpl=`picscount`]]`]]
Chunk "picscount":
[+maxigallery.picscount:math=`?-1`+]
Thanks everyone for your help!