[maaku]

> Early in the section I mentioned that there is a natural way of reducing the variance in time required to validate a block of transactions. If that variance is reduced too much, then it creates an interesting attack possibility...

You've described a 51% attack. The security model of bitcoin assumes these are economically infeasible to mount.

> Suppose Bitcoin mining software always explored nonces starting with x = 0, then x = 1, x = 2,\ldots. If this is done by all (or even just a substantial fraction) of Bitcoin miners then it creates a vulnerability. Namely, it’s possible for someone to improve their odds of solving the proof-of-work merely by starting with some other (much larger) nonce...

No, this does not improve their odds. No miners are scanning the same ranges, because the block header is different for each miner (due to different coinbase transactions and timestamps, if for no other reason).

[michael_nielsen]

> You've described a 51% attack.

No, this isn't related to 51% attacks.

Your other point I agree with; this is also discussed in the post comments.

[maaku]

If Alice is able to maintain a fork at equal length to the main chain, that's a 51% attack.