What's important to him is to be able to serve his Coda Docs to his paying clients under his domain name. That's it. My MVP must do that, it's what the customer bought. If it doesn't do that, I'm not picking that option. If I don't think I can build what does that with the tool, then I won't use that tool.
Thankfully, clarity comes in that there's something that I have experience in and can serve this purpose. Flask. Yep, what better way to build a MicroSaaS than a microframework. And "micro" also means production-ready. Really, we're overcomplicating serving HTML over the net. Serve that. Serve Coda embedded in iFrame.
I know how to build that in Flask. I even know how to expand functionalities from that in Flask. But that would be getting ahead of myself. I'd also be getting ahead of my customer. The goal is to deliver the MVP and I will handle the serving iFrame first. Then onto serving that over a custom domain. Time to draw out a few flows to serve as guidelines.