Question about spam attacks From my understanding, an unconfirmed transaction can be accepted into the mempool provided that it has a fee that is generally at least 5460 satoshi, which is set as default in the current version of Bitcoin Core (assuming input=148B, output=34B, and relay node hasn't changed default settings). If a spammer sent a transaction like that, wouldn't it most likely not get picked up from the mempool by any miner (because of low fees) and thereby be returned to the original spammer at no loss? Couldn't this happen for a large number of similar transactions?
yes, but then the "spammer" hasn't really accomplished anything.

essentially with spamming its either
1) you paid to get a "spam" transaction included in a block
or
2) you paid nothing to get it in the mempool

not really spam either way at this point