Those are great questions. Let me take a stab at them:
Can Web Monetization be used during server-side rendering?
Not really. Or at least not while it's being shimmed by an extension. The future browser implementation can send those details on the HTTP request. This way, the extension isn't part of the initial page request, and can only access the contents of the tab it after the GET request initiated. Monetization only starts after the page is loaded, rendered, and a meta/link tag with a valid payment pointer has been found on page. The delay can be a few seconds until that happens. What you're looking for here might be more along the lines of returning a 402 Payment Required status.
If yes, can Web Monetization be used using only HTTP requests server-side?
This use case doesn't feel like Web Monetization - streaming payments for content continuously while I consume the content. It feels like a single time payment for content. There are other options for that use case. Web Monetization wasn't designed for that use case.
Where's the crypto???
There's no crypto. That's why you're having a hard time finding it. Web Monetization uses the Interledger network to send the money. I understand the "ledger" in the name makes some people think of crypto, but in here it refers to what a ledger used to be before it was appropriated by crypto: a book of accounts that records transaction. By that definition, a bank, a mobile money provider or a financial institution is a ledger. Even Starbucks is a ledger (you have an account with them, it holds some of your money, and it records your transactions). Interledger was created to provide a bridge between all these disconnected places where we store money. There are a bunch of reasons why this is happening, worthy of a whole blog post on itself. Actually, I have a talk that goes deeper into this - youtube.com/watch?v=04-pGMMQZMo
Hopefully that answered some of your questions, and let me know if you have more. I think what you're trying to do here is super cool, and I'd love to help if I can.
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Those are great questions. Let me take a stab at them:
Hopefully that answered some of your questions, and let me know if you have more. I think what you're trying to do here is super cool, and I'd love to help if I can.