So assuming that the format stays unchanged from previous seasons, and the contestants do a previously performed song at T3, and 1-2 previously performed songs in the finale, how does it look like this will play out?

I think strategically any contestant should give their absolute best performance at T3. E.g., Melinda made a critical mistake NOT doing My Funny Valentine that week last season (undoubtedly she was saving it for the final). So if Arch does "Imagine" and Cook does "Billie Jean"/"Hello", and assuming they both advance what does that leave them with?

Cook can obviously sing the other of those, and the 3rd could be "Eleanor Rigby" (although I doubt he would do 3 covers of covers), but I think most likely he'd do either "Always be my Baby" or "Little Sparrow" (both original arrangements).

David A. repertoire is a little thinner. The best of the rest are probably "Long and Winding Road" and "Angels." But between those and the cheesy pop ballad, he won't really show much versatility or originality. It hurts him that he doesn't have any knockout performances of upbeat songs. He might be best served singing something new (like Blake did last year), although then he risks doing a bad job.

Obviously we could still get "homeruns" in the next couple of weeks that could be repeated, but with the exception of McPhee doing OTR back to back, the contestants usually pick songs from earlier in the season.

I think you can add "Shop Around" to Archuleta's list, not because it was one of his best performances, but because it was one that showed some versatility early on. Kind of upbeat and different than most of the slower stuff he's done since then. It wasn't bad from what I remember.