If change addresses are not automatically recovered from a backup, that seems like a serious bug/shortcoming in Bitcoin Core to me.
A couple of questions.
Remember you MUST have the public keys available to you (you can find this in the explorer, ...
You write that you need the public keys, but the instructions never use them. When you wrote "public key", did you mean "address"?
How do you find the public keys (or perhaps addresses) generated by the wallet in an explorer?
Public Key = Wallet Address for this context.
The REASON you should have this on hand is so you can be clear how much $$ you have in those.
To find your change address, you can open the explorer of your coin and track the change addresses used when you sent money from your original wallet address (which you know because it's present in Wallet 'A').