Agree that adding extra connections would be a negative. Which is why my suggestions don't involve any extra splices. Nobody has commented on any of them specifically, so I don't know if any of them would be considered NEC compliant. Of course, what matters is what your AHJ would consider NEC compliant--Dennis's comment suggests 312.5(C) is routinely ignored in your area.
I believe the North Carolina 6' outdoor allowance for NM is only within conduit. Basically that outdoor conduit runs up to 6' in length are considered dry locations.
Right, that would only work for NM in NC.
Well, if it's 20 small NM cables, and clamps can only handle 2 cables each, then yes, I'm proposing drilling 10 holes in the junction box/enclosure/wireway/whatever, and one big hole to attach the large PVC nipple (< 24"). So if the existing PVC goes through the rimboard, the box needs to be recessed inside a joist bay, could be a bit difficult if the joists run parallel to the rimboard.
No, I don't see the problem. The wireway or junction box would be in the crawl space, there'd still just be a single 2" PVC conduit passing through the siding.
A shared EGC only need be large enough to suit the largest circuit's requirements; a #10 in most cases.
You can combine circuit pairs into MWBCs, as long as you tie breaker pairs, but not one large neutral.
Yeah, late to the party
That's what's been said above is needed in order to be NEC-complaint. But fellow Tarheel Dennis, intimately familiar with our code variations, believes it'll probably fly without doing that (if I understand correctly), especially since it's a straight replacement, where, for example, upgrade to AFCI is not required.