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This assumes that one un-compromised round of Darksend is enough.
Enough for what? Could you elaborate please?
The problem is that this will not work with a partially compromised masternode network. I could very likely still end up with and effective Darksend of 1 round.
I'm trying to get my head round this
While I keep trying, maybe you could illustrate with an example of some kind.
Let us say I face an attack that will work against 1 round of Darksend but will fail against 2 rounds of Darksend. This could be the Sybil example I quoted above. If the attacker has also partially compromised the masternode network, then I need a sequence of 2 un-compromised Darksend rounds for protection from this attack. In this example sequence 1 will not work
1) Honest Malicious Honest Malicious Honest Malicious
but sequence 2 will work
2) Malicious Honest Honest Malicious Malicious Honest
because of the bold part. So it is the probability of the sequence of n honest masternodes in the chain that matters, and this is much lower than the probability of a single honest masternode in the chain.