[ricbarbour]

A. When the Nissan rating of 60Ah was calculated, what were their top and bottom voltages? Did they go down to 2.8v?

No one knows. It is considered "proprietary information" by Nissan. If you're lucky a blogger or DIYer might have posted their own test results.

2018 Nissan Leaf battery real specs - 🔋PushEVs

The specs of the battery cell used in the 2018 Nissan Leaf are pretty impressive.

pushevs.com

B. At 8.5 years old this seems pretty good even at only 54Ah, how do we find appropriately matched replacement cells?

You're not supposed to "find" matched cells. You're "supposed" to buy a finished pack.

If you insist on DIYing you will be forced to buy a stack of surplus cells and exercise, then match them yourself. Will Prowse has made many YT videos showing how he tests various lithium batteries for capacity, he ended up buying special testing equipment--most of it comes from obscure Chinese companies apparently.