OK, more news on the bizarre behavior front. While I 1st detected the anomalous behavior — unwarranted appearances of the message "This script cannot be found or has been deleted." — while clicking on a button, I've subsequently discovered that it turns up under other circumstances as well:
 • opening the file, tho I've been able to prevent this problem by disabling the Startup script
 • changing from one layout to another using FileMaker's drop-down menu of layouts — but not in all cases
 • quitting FileMaker Pro, whether selecting it from the menu or using ⌘-Q — but, again, not in all cases

As before, all of the scripts seem to be doing exactly what they're supposed to, whether activated from the "Scripts" menu, the "Manage Scripts" dialog, or by clicking on buttons they're attached to. Same deal with the above 3 operations. They do exactly what they should, but often preceded by that annoying and seemingly meaningless message.

A clone of the problem file exhibits the same behavior, even without any records.

Other FMP files do not produce any problems at all, so it's gotta be some kind of corruption in the one file, rather than misbehavior in the program as a whole. AFAICT, FileMaker is just throwing up the "missing script" error message because it's somehow confused and that's the 1st error message that comes to hand; the underlying flaw appears not to be exclusively related to scripts or the buttons they're attached to.

So now I find myself in damage-control mode. As I said at the outset, I had just finished a 10-hour work session on this file when this problem materialized, and I sure don't want to have to spend another 10 hours just recreating the same work. Does anyone know whether it's safe to start with the previous day's backup copy (and not with the copy itself but with a dupe of it) and then copy and paste stuff from the problem file into the new working version? Or am I really stuck having wasted that 10 hours?