For an additional quick test I forwarded a signed+encrypted mail from KMail as attachment, the mail containing the attachment was a) signed only, b) signed+encrypted.

Copying the file to the new location works as proposed in the description, tested with update from 3.2.2 to VS-Desktop-3.2.93.33-Beta

In Debugview I see, after only opening the group configuration dialog:

Migrating kleopatragroupsrc to new location worked for update 3.2.2.0 to VS-Desktop-3.2.93.33-Beta on Windows.
kleopatragoupsrc in old location is not deleted, but a copy is written to %APPDATA%/gnupg/kleopatra

Tested with VS-Desktop-3.2.93.33-Beta, works!

Tested with VS-Desktop-3.2.93.33-Beta:

Tested with VS-Desktop-3.2.93.33-Beta, where everything necessary is backported:

current plan after discussion today is as follows:

works in VS-Desktop-3.2.93.33-Beta

afaik "Valid Until" is actually available for keys coming from WKD

Very good suggestion! +1
I believe Werner and Alexander will agree, too, as we talked about that tab the other day and that showing only the last certification would be preferable

If this is easy to solve and backport, I would like to have this in the next release, as we did a lot of column related changes in other tables already for that release.

Suggestion for the "placeholder" screen:
Only show "Please insert a compatible smartcard." And then below: "Known supported smartcards are listed at https://gnupg.com/kb/smartcards.html".

Did a quick manual test import and encryption/decryption with VS-Desktop-3.2.93.1-Beta with the relevant test-X509 certificate.
Works as expected.

First: we should do something about the placeholder text, too. It is a bit misleading ("But it says Netkey cards work, why doesn't mine?"…)

As there was some discussion in the ticket: what has been implemented? How can it be tested?

to document the answer to my own question: The serial number is unique for the manufacturer, e.g. Yubico. But serial numbers of devices of different manufacturers might have the same serial numbers, although it is unlikely.

with gpg4win-Beta-41:

Status is testing for 2.4, no backport yet for 2.2, so there it stays in the backlog column

checked with Gpg4win-4.3.2-beta41:

We decided how it should be implemented.

as this is a regression, I would like to have a fix in the upcoming release

looks fine, gpg4win-Beta41

Try it. It does not do what you think ;-)
You will revoke the certification of the User-Id and not remove that one certification. I should have taken a better example for the screenshot:

Icon is there in GnuPG-VS-Desktop-3.2.93.32-Beta

tested with Version VS-Desktop-3.2.93.32-Beta

works

done (gpg4win -Beta41)