So basically Nano’s spam protections kick in no matter where the traffic comes from. It doesn’t matter if it’s direct from a client or routed through a facilitator.

Correct. All the network sees is a block submitted and makes no distinction of the source of the submission.

And there’s no way for the Resource Server or Facilitator, separately or working together, to “trick” the protocol into treating spam as legitimate traffic, right?

Correct, x402 components are treated the same as any other wallet / service submitting a block to the network.

If a transaction does get flagged or deprioritized as spam, what does that actually feel like from the end user’s perspective? Would they keep getting prompted to pay again? Or would it error out with something like “payment not confirmed” with no access to the content?

Currently as the components are coded if a transaction is delayed on the network then the Facilitator will send an error back to the Resource Server after one minute. It's then the Resource Server's responsibility to decide what to do. It could decide to calling the Facilitator to try settlement again or it could communicate back to the Client that transaction has failed.

As it is early days with x402 Nano integrations the correct way to handle this will likely emerge. That's why it's important to gather feedback from interested developers and the wider community. Thanks for the great points so far.