I'm wondering what would happen if there was an SNS topic having messages written to it, but for a period of time, there is no SQS queue. Let's say there was a container which normally was subscribed to the SNS topic to handle such messages, but it crashed and burned and spent 10 minutes getting resurrected; what would happen to any messages written to that topic, during which there is no queue? Do they disappear forever, or do they wait politely until some queue comes along, subscribes and picks up said messages?
They disappear forever.
SNS cannot know that some subscriber wants to subscribe but simply cannot right now. The topic either has subscribers or it does not. All current subscriber get the message, all future ones do not.
If you have subscriber but the delivery fails there is some SNS specific behaviour in regards to retries: https://docs.aws.amazon.com/sns/latest/dg/sns-message-delivery-retries.html