Explain how unused addresses in wallet.dat have no balance when mining occurred for many months
Unused addresses are unrelated to mining activity. Even if you were mining, you will still see many unused addresses. The Bitcoin earned from mining would be associated with a used address.
Bitcoin Core by default will generate 1000 (previously 100) unused keys to be used in the &quot;keypool&quot;. The keypool is the pool of unused keys and is used as a lookahead. This is useful when you restore a backup. The unused keys you are seeing are this keypool.