Would it be safe to assume this is the worst of the five at the mating surface?

Either way, the evidence doesn't support detonation as the bore is too clean and brite, I would lean more towards inherent casting stress and harmonics.

What does the rod bearing shells look like on the back sides? brite or dark? are they still snap fit or do they drop out of the rods and caps?

Detonation can obviously kill parts, but its also going to leave a good bit of evidence behind, we haven't even discussed piston decks, ring lands, chamber coloring and so on.......lol

I would forge on with the RB block for your objectives, just make sure the balance is done by a reputable shop, keep the balance neutral and if you can swing it ($) switch to a 4/7 & 2/3 swap cam, one of the best kept secrets for reducing harmonics, the cores are not cheap but it is good insurance.