I use quite short documents (like scenes) in chapter folders. When I try to make a snapshot of a chapter, I have to do it (so it seems, maybe I’m wrong) for any contained document individually.
I’d like to see here (and for other actions like changing state or labels) that all selected items would be considered.
Have a look at §15.8.1, Creating Snapshots, in the user manual PDF, for better options on taking snapshots of larger groups of documents. The yellow tip box in that section may also be useful to you. I like to give bulk snapshots a common name so I can know there may be other snapshots related to an edit elsewhere. The Snapshots Manager tool makes it easy to fetch a list of items that have a snapshot of a particular name.
Another tip is to check out §15.8.5, a few pages down. The second bullet point, which can have snapshots created for you on all edited documents automatically, is a different way of thinking about it as your snapshots come after the edits instead of before—but if used routinely the effect is much the same. A big advantage here is that only the snapshots you actually need are created. Blanket bulk snapshots on a whole section can result in snapshots you don’t actually need unless you really do edit everything every time.
Personally I use a mix of both! It depends on the situation, but I like having the safety net of the second option. It is a little peace of mind once you form the habit.
I’d like to see here (and for other actions like changing state or labels) that all selected items would be considered.
See §10.4.7, Setting Metadata to Many Items. 
Many thanks! I’ll check it out …
Following up on the question how to perform actions on a set of selected documents, for example for the whole draft including all documents: Is there are way, say, to tidy up multiple spaces on a selection of documents? I tried but find no solution, but I’m still quite new to Scrivener and maybe I just don’t know the right workflow?
I know that I could use regexes for (dangerous ) project-wide replace operations … but that’s a hassle. I would really need a fast way to repeat such actions for any arbitrary selection of documents …
Well, only as dangerous as the backup you don’t have. Have one of those and it’s not really dangerous at all. File ▸ Back Up ▸ Back Up To... is a command I use an awful lot of, whenever I do something that might have no recovery. Set it to create the copy into the same directory as the main project, with a name that describes why it was made, and zip compression enabled. Now you can be free to make a serious mistake.
If it happens, what I do is use Project ▸ Project Settings..., set its backups to be excluded (so we don’t automatically back up the damaged project when closing it), close the project, delete it in File Explorer, and then extract a fresh copy from the zip file. Now it is as if it never happened.
As for the broader question though, there are indeed some blind spots that could be improved. Most of the text editing level commands must be run in the text editor (obviously) but in doing so that means they can only impact one thing at a time. Scrivenings session was meant to bridge that gap as a way of viewing many things at once, but unfortunately technical limitations have made it difficult for it to actually work that way (hence, Ctrl+A only selects up to the item level).
Great backup workflow!
And for the other question: I thought so (but still hoped for any cute trick I didn’t find. :- ) ) So the best tactic here would be to add a regularly Ctrl+Space to the usual editing process … I’m not on a Markdown workflow so multiple spaces are something that affects me …