The original manuscript has the subordinate conjunction when, which Oliver Cowdery accidentally copied into the printer’s manuscript as after, perhaps under the influence of the following prepositional phrase “after their manner”.
Elsewhere in the Book of Mormon text there are numerous occurrences where the subordinate conjunction when has the meaning ‘after’. Here are two examples:
So there is nothing wrong with the original use of when in Alma 31:23.
Summary: Restore the original when in Alma 31:23 (“now when the people had all offered up thanks”), the reading of the original manuscript.