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the tone you've taken here is irritating and is likely to cause experienced people to ignore your messages in the future if you continue with it.

My apologies. Did not intend to irritate. Just trying to understand this problem better. And many thanks to you for the time you're taking to go through it here - very much appreciated.

I hope you don't mind if I continue:

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once the honest network is observed its like the node never saw the forgery. When you start talking about "check-pointing" based on that chain the situation changes and you get the attack

This is where I'm losing you. Yes there may be a checkpoint, but highest total difficulty still wins out. If the highest difficulty chain conflicts with a check-pointed one, surely the client should go with the higher difficulty one, as you say

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chain will simply be unwound and replaced, so giving it that extra data is harmless, once the honest network is observed its like the node never saw the forgery

. The checkpoint is merely a mechanism to avoid verifying very old txs. But if I see a competing chain with higher difficulty, I ought to go with that one, whether it has a checkpoint or not.

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I can mine 80 blocks in a row at height 100,000 trivially in a few seconds myself

Granted. But if you go back and do that, the chain you create will not have difficulty of the canonical chain. So even if I see yours first, again, so long as I eventually see the real chain I will ignore yours. This check-pointing mechanism would have to start from the current head if it wants to stay valid. We could check point block 100,000 by submitting today a tx with the hash of that block. If you try to actually create that checkpoint further back by forking around block 100,100, say, you will not be able to create a chain on par with the current difficulty. So despite your checkpoint, I will still ignore you, even if it means I have to hop on a chain that starts from satoshis genesis and has no checkpoints.

Is this not correct?