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You should split up your other, unrelated questions into separate questions - this site isn't really designed for multiple independent questions in a single post
@MeniRosenfeld True, but I figured it was best to give the overall approach rather than specifics - not all non-HD wallets had keypools, iirc, although Bitcoin Core and other commonly used ones generally did
Not-modern wallets did the same thing, just without a single root key - For each address in a non-deterministic wallet, the wallet software would generate a new, random private key.
@PieterWuille True, but I suspect the OP was referring to the nonce issues, since they specifically called out faulty clients, which is how several keys were leaked in the past
