I tried but couldn't reproduce it any more. Therefore setting it to invalid.

Before making subtickets for each application: I wonder if it is not all Kleopatra anyway? Isn't the security approval dialog basically Kleopatra?

The equivalent for invalid S/MIME certificates are not-certified *PGP certificates.
(Valid/invalid are not ideal as technical terms as they have a broad general meaning, too. I hope my usage here is correct ;-) It is what I gathered from an explanation given by Werner.)

Note: The invalid revocation certificate: Bad signature - rejected line is also shown on vsd 3.3.4, gpg 2.2.53 @ win10 (but revocation works).

feedback of @mmontkowski needed

Invalid certs (as stated in the status column in Kleopatra) are mainly S/MIME certs (e.g. with missing root cert, CRL check failed, etc). I haven't seen invalid pgp certs yet (might be e.g. very old ones with missing self signature).

Invalid and expired are different cases.

Not a good idea. Because then the user will open it with the browser and the browser loads all kind of additional data including drive-by malware. If HTML *mail* is shown by a MUA no links should be followed to keep information and the fact that it was read confidential.