https://www.screamingfrog.co.uk/seo-spider/

It is not a test that determines all images on that page.

On mobile, on that specific page, only one image is actually visible on that website in the initial viewport, so it correctly shows one result there.

The website is https://www.francescocostantino.it, and the specific page I'm concerned about is:
https://www.francescocostantino.it/francesco-costantino-artista-contemporaneo/

To answer your question: only one image is indexed.

But here's why I don't think this is just "Google being selective about which images it indexes": I ran the page through Google's Rich Results Test (which uses the same rendering engine as Googlebot), and in the rendered screenshot only 1 image out of dozens in the gallery actually shows up, the rest stay as blank/placeholder. You can see it here:
https://search.google.com/test/rich-results/result?id=xXK7lCrVXxZVIHqsarDoMg

That's a different thing than "Google chose not to index some images": it looks like during headless rendering, the src/data-src swap simply isn't completing for most images on the page, not just a curation choice on Google's side after successfully seeing them.

So my question is: is there something about how/when the JS swap fires (scroll event, IntersectionObserver, container size calculation via getBoundingClientRect, etc.) that could fail to trigger during a headless render where there's no real user scrolling? That would explain why the rendered screenshot shows almost nothing, even though on a real browser with a real user everything loads fine.

When I coded this, I realized this is a good approach because I saw many instances where the small preview image would load and then the bigger resolution would load afterwards. I did that to prevent double loading of different sizes.

So this was more of a performance decision.

There is no setting to disable lazy loading just for one part of the website.

You can, however, disable lazy loading for the whole website and also check this setting here called "Never show resized versions of images".

I think this setting is in Lay Options in the lazy loading section or in the image section.

Okay, I just checked, and I realized even with those settings, it will still load the images via JavaScript, and it will still have a 1x placeholder in the original markup.

So when Google indexes your website, it lets JavaScript run, so Google can actually find all of your images.

The thing is, over the years, sometimes people have come up with the same concern: "Oh, my images don't all get indexed by Google." I think the issue is not that Google can't see your images. The thing is, people expect all of their images to be indexed by Google, but the truth is, Google does not index all images. It kind of decides on its own how many images it indexes and which ones, and that's something that you can't control.