Her people were a mixed culture which compromised Jewish with pagan culture -- the thing you don't like about Christianity. They had a copycat religion with modified laws (or interpretations), and they disputed with Jews about some big ticket items as can be seen in Jesus conversation with the 'Woman at the well'. (John 4:9,20) It appears that they wanted to be accepted as Jews as they were, so they were trying to modify Judaism, hence they were accused of opposing the Lord. Besides in the passages above, its not David who makes the distinction of dogs but the Lord who makes it between the Israelite and the Egyptian of the time of Exodus. The enemies aren't David's enemies but the Lord's enemies, and that is the most clear in the story of Goliath where David attacks in the name of the Lord. If these are only David's enemies, then why bother praying to the Lord to defeat them? Its reasonable to think that the Lord's enemies were called dogs. There is no need to defend him until you've established that he has not followed the golden rule. He only refused to do anything until she showed that she was not opposed to the Lord. Its clear that he had no problem with her lineage but only with enemies of the Lord, dogs. Therefore, the passage in question may not be a late addition.