With the work done for PGP signed-only messages, it was not very hard to add S/MIME signature verification and S/MIME decryption anymore. You'll need to import one or more keys into Android to use this. You can do this via the advanced Android security settings, which you can conveniently access with a button from the privacy settings of FairEmail.
Edit: since the certificate chain is verified, it is enough to show the name of the signer to see if the signature was okay. @gc01 is this alright in your opinion?
Please report any problem you discover, so it can be fixed.