The idea that the "non-returner" concept was misconstrued, as I read the passage you quoted, is based on the assumption that non-returners arise as formless beings. If non-returners do not arise as formless beings, then the two questions ("has the non-returner concept been misconstrued?" and "can we make sense of formless beings?") are independent of each other and the latter can then not be used to answer the former without further support.
Yes, that's so. The latter question of whether we can make sense of formless beings is the only one I am interested in here, and the non-returner aspect is only provided because it is the context in which Gombrich talks about it. That is, he might well be wrong about non-returners, but his doubts about the formless realms are interesting, whether or not they are misplaced or misused.
If formless beings don't make sense, then how are the formless absorptions to make sense?
Presumably, by understanding them as perceptions of formlessness, or perceptions that are without form. That doesn't mean that a person so "absorbed" (i.e. having the meditative experience) has lost their physical body, or what differentiates it from other material objects. I'm not sure of the Pali word, but I assume that the difference between mental absorption ("I didn't hear the rain because I was absorbed in my book") and physical absorption ("The ink was absorbed by the blotting paper") is relevant here.
Aren't the experiences of beings in the formless realms effectively defined in terms of the experiences of the formless absorptions (as in my previous quote from DN 15)? It seems that dismissing the possibility of formless realms requires similarly dismissing the possibility of the formless absorptions.
They might be defined in this way, but if formless realms are different from formless absorptions, there is nothing about the experience of the absorption which determines what the "realm" is. We could dismiss (say) the possibility of Utopia, while believing that one might become absorbed in ideas about it. But I don't think anyone is dismissing the possibility of formless realms. I'm merely asking whether we can make sense of it.