Using the index numbers it was possible to see what happened. The online and offline instances had indeed generated new addresses of different types at different times. For example, the signer had a batch of 84 'extra' addresses allocated as P2PKH where the onlines had allocated batches of 40 and 32 addresses.

It does make checking a tx slightly more cumbersome compared to when there was just one list in-sync across the machines, but with the forced consistency check and the ability to choose an address that does also appear on the now-displayed signer's list, all is well.

As to the initial non-recognition of a wallet address, after the usbs in/outs and reboots, I couldn't reproduce it and am putting it down to a glitch in the matrix or "my bad" somewhere along the line.

Many thanks for the quick help.

The only other thing that came up is that it doesn't seem possible to choose P2SH-P2WPKH as a change address - is that waiting for Core's 0.16 improved segwit?