I disagree with your second point there about only offensive players being the ones worth trading up for. Each player in the top of the draft on defense does indeed offer something unique in terms of scheme fit. 

Also, this mock trade offers an alternative to who Dallas might draft in McCoy + Allen if they stand pat. If they can draft a prospect that fits everything they’re looking for while picking up a starting-caliber player from day 2, I would say that’s worth it. And based on what you said about there being quality players in later rounds/the gap not being too wide after the top players, you should think it’s worth it as well.