Is it permissible to amend an article after submitting it to a competition—such as by adding to it or deleting sentences—knowing that the article is about the Messenger, may Allah’s peace and blessings be upon him?

Whether or not to change the article participating in the competition is up to the organizing body. If they stipulate no changes, then adherence is obligatory. If they permit changes, then there is no impediment. The inquirer can seek clarification from the organizers.