Late answer because it took me some time to figure out how ZeroMQ works.

Quote from: tiagocs on November 12, 2020, 10:27:27 PM

1 - The default high watermark for bitcoin core is 1000. If I understand correctly this means that up to 1000 raw blocks can be stored in memory (send buffer), which could translate to approximately 2GB, if there is a subscriber who forgets to read the notifications. This may lead to a crash due to insufficient memory in many machines, so it would be safer to set it to 100 and that would still allow the notification subscriber to be ~14h late in reading them, correct?

Well this will only be a problem if you subscribe to ZMQ notifications directly because there is an RPC call that can fetch the blocks that you missed. So ZMQ won't send you the missing blocks but you can pass the last block hash you received from ZMQ to the getblock RPC call, it'll give you the height which you can increment and pass to getblockhash to get the others, stopping once you reach the block hash in the new ZMQ notification.

Quote from: tiagocs on November 12, 2020, 10:27:27 PM

2 - It seems like raw block notifications are not published twice for the same height. An issue was just published on the bitcoin repository, but there are no comments yet: https://github.com/bitcoin/bitcoin/issues/20376 . Is this expected behavior or a bug?

You got the answer in the Github issue, if the new block at that height was already in the active chain, then there is no notification, but you can subscribe to the "sequence" topic as a workaround to get notified again for the height, discerning the information from block connections and disconnection messages.

You currently need a node running a development version of bitcoin core to use "sequence" as a new release hasn't been made since the PR that merged it.

https://bitcoin.stackexchange.com/questions/91678/is-it-possible-to-catch-up-missing-blocks-with-zeromq-if-script-was-down-for-a-w
https://github.com/bitcoin/bitcoin/blob/master/doc/zmq.md#remarks