> Followup question: how does a node coming online know not to trust China's (longer, censoring) chain? It wasn't online to have the transaction in its mempool, so it doesn't know to check for it in the longest chain.

> I think it would need to check all candidate blocks with lower heights to see if their chains contain any transactions that aren't in a longer chain.

Yes. You wouldn't need to store all these, though, you just sweep them up once you find the transactions included in the longest chain.

> What happens if I mine off of a very old block and include my own transaction in it, and present it to you... how do you distinguish between what I just did vs the longer chain having censored the transaction this whole time?

Broadcast all transactions you have which aren't already in the longest chain to the network, and wait for them to include them.

I'm not 100% sure, but I think transactions get rebroadcast automatically already until they're included.