Yes, as I understand the first scenario, it’s a static way possible for a few devices, where each has a folder to each other device for that purpose. With an increasing number of devices, this needs then an increasing number of folders between all devices to allow a dedicated one-time sync of files that way. This becomes fast very unhandy.
Yes, with the second scenario a sync-share of a folder is possible as you described it. Your one-time sync is then
(1) “A” creates a folder and copies files into it
(2) “A” adds the folder to syncthing and offers it to “B”, “C”, etc.
(3) “A” clicks on “Advanced” and choose “Send Only” as “Folder type”
(4) “B”, “C”, etc. receives the ““A” wants to share folder <…>.Add new folder?"”
(5) “B”, “C”, etc. accept “Add” to access the folder
(6) “B”, “C”, etc. create a new folder to link it to that offer
(7) “B”, or “C”, etc. copy/move the folder or the files to another place
(8) “A” may see now “Out of Sync” for the folder and “Up to Date” for the devices which is for an one-time sync not a proper information
(9) “B”, “C”, etc. delete the folder in syncthing
(10) “A” has no good information about whether it has been delivered to everyone and keeps the folder for an undefined time.
(11) “B”, “C”, etc. who has deleted the folder in syncthing gets again the invitation for that folder they already got. They can decide “Add” or “Later”
(12) “B”, “C”, etc. who already got and copied the folder have now to decide whether this is an new invitation or an old one, especially, if a some folder are offered that way
(13) Everyone might be on a way to confusion
I suggest
(1) “A” creates a folder and copies files into it
(2) “A” adds the folder to syncthing and offers it to “B”, “C”, etc. for “One-Time Sync” and expires at …
(3) “B”, “C”, etc. receives the ““A” offers the One-Time share folder <…>. Accept?"”
(4) “B”, “C”, etc. accept to access the folder
(5) “B”, “C”, etc. point to a folder into which the “One-Time Sync” will be copied (it already has a name given by “A”)
(6) “A” gets the information that “B”, “C” got the folder and after everyone got it (or the expire date arrives), syncthing deletes that “One-Time Sync” share at “A” (but not the folder)
(7) Whatever “B”, “C” are doing else with the folder after its delivery has no further (informational) effect on “A” nor do “B”, “C” etc. get the invitation another time