Hmmm...
Both of these sites agree with me that “Saved by the bell” is a boxing term. The second site does mention your coffin theory, in order to debunk it.
https://www.merriam-webster.com/words-at-play/words-phrases-from-boxing-wrestling
https://www.phrases.org.uk/meanings/saved-by-the-bell.html
Enjoy.
Im sticking with my theory. Although your theory has some legitimacy to it also.