what will be executed first if I have a mailbox with spamassassin & email forwarding both activated: will the email be forwarded after spam treatment or before?
I don't know the situation with the Plesk/Generic SA for sure, but with the 4PSA SpamGuardian tool if you have forwarding enabled then there is no Spam or AV checking on the forwarded email.
I would assume the situation is the same for the Plesk-supplied option due to the way the email is handled.