Right. So, having not signed out before the swap seems to have royally eff-d both systems.
I returned the SSD back to the MBP to see if I could return to ground zero. No dice. Same issue persists. Signed out, back in to iCloud - nada, there's no getting iCloud and iMessages working again.
The console shows:
3/22/16 4:43:25.272 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:28.568 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:31.844 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:35.110 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:38.401 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:41.773 PM apsd[100]: Certificate not yet generated
3/22/16 4:43:45.066 PM apsd[100]: Certificate not yet generated
Which is specific to apple push notifications and googling seems to indicate that it's a big part of the issue, but no clue on how to resolve that.
There's also a myriad of other errors.
Sigh.