I think a lot of this debate is going to be solved by what Dallas does in FA. I'll be upfront; I think Dallas' best course of action is to sign Carr in FA and then draft DeCastro with their first. The reason for this is two fold. First, it's no secret that the secondary is the absolute weak link of the team. There needs to be an upgrade there, either through a proven commodity with a high cap charge, or by using your first round pick on a DB. I'm hoping for the former, because I see DeCastro as a blue-chip prospect in the draft. 99% of the time, blue-chip prospects do not get out of the top 10. It only happens if there is character/injury concerns (See Dez Bryant or Randy Moss) or the player is playing an under-valued position; OG, C, TE, FB, the occasional Safety, etc. DeCastro plays one of those positions, so there's a somewhat good chance he'll be available when Dallas selects. You win games, and championships, by getting as many elite players as you can; DeCastro is more elite than any of the DBs in this years draft, IMO.
That all having been said, if Dallas ignores the DB position in Free Agency, or signs a moderate to high-priced OG, you kind of have to draft a Corner with your first selection, even if DeCastro is still on the board. Yes, it is drafting for need, which is not a great thing, but that black hole in the secondary needs to be filled. There's just no way around it, really.
Finally, should the Cowboys sign neither a Corner or Guard in Free Agency, I think that's pretty indicative of failure on the Cowboys' part. You can hope and pray that a 2nd round selection pans out immediately and can be a starter, but again, you're setting yourself up to draft for need, and theres no guarantee the position you need to fill will have a decent player ready to be picked.