There is a limit on how much a block can sigop (set to 80000?). There is also a limit on how much a single transaction can sigop (set to 16000?). It thus seems possible (and if it's not possible, the remaining of this question is void, but then how is it prevented?) that an attacker submits five special transactions to consume the whole block sigop limit. If such attacking transactions are using very generous fees per byte, if the miner chooses which transactions to include in a block based on fee per byte metric, it would include the attacking transactions in a block, getting relatively huge fees per byte, but not filling the block because of sigop limit. This would mean that the miner could get more in fees if it avoided attacking transactions, but the question is whether such selection avoidance is implemented by default.
I'd like to know "how an attacker would be able to work out the private key of the extended public key, and therefore get every key that can be derived from that, hardened and non-hardened." should they manage to "1) gain access to the extended public key and 2) one of the non-hardened private keys that was derived from it"