Questions about particular use cases (integrity verification w/o private key, add E flag to primary key, import secp256k1 key)

[Robert J. Hansen]

> If I have the public key of the recipient, I should be able to tell that
> a message was encrypted for that public key, except I am missing the
> private key to decrypt it. If I can check the message format I should be
> able to check this as well. How would I do this with gnupg?

--list-packets

>>> 2. Is it possible to have just one key (the primary one, no subkey) with
>>> E flag also (S,C,E) -- I know this is not recommended but this is a
>>> particular use case and the risks are acknowledged. I guess gnupg will
>>> not allow you to do this by default, but is there any magic that can be
>>> done?
>>
>> Yes.
>>
> 
> How? I tried in expert mode but didn't manage.

--expert --full-generate-key

Options 8 or 11 should work for you.  Haven't verified it.