I don't mind if you think that these are "cute" or even "plausible"
but I don't think that any of them are historically correct,
and I think it is a bad idea to circulate phony stuff and suggest
that it might have some basis in truth.

"Dead ringer" is U.S. slang; the OED's earliest cite is 1891.
It has nothing to do with ringing a bell.  It's about a fraudulent
substitution or a close resemblance.

"Threshold" is one word with one h, all the way back to Anglo-Saxon
when it was spelled prex-; and the etymologists do not know what
the origin of the "-old" is, but nobody thinks it comes from "hold."

"Trench mouth" was identified in 1896 by a Frenchman J. H. Vincent
(1862-1950) and described in Lancet in 1904.

"Saved by the bell" is boxing, of course.

Don't let a poverty pimp tell you that "handicapped" is an insult
to the disadvantaged because it means "beggar with cap in hand."
It means no such thing.  Like "ringer" and "saved by the bell"
it is sports slang.  (but "handicap" at least goes back to Britain
a couple of centuries ago)

On this list it would be a better idea to have a competition
for the cleverest fake etymology.  I will give one from the 1940s,
not as a competitive entry, but to illustrate what I have in mind.

"Lunatic": from "luna" = moon, "attic" = top story,
hence a lunatic is one who is moony in his top story.

Another quasi-architectural example:
"auditorium": from "audit-" to hear, and "taurus" bull.