Coda is always moving quick with updates and you caught us in the middle of two things happening at once.
First, good news is our API is getting an update and this will address the Sections → Pages issue.
Second, this feature in Cross-doc is getting updated in the next weeks to use the new version of the API so it’ll soon be “Page” and not “Section” as well. For now, putting your cursor anywhere outside of a table on the canvas should give you the relevant URL to use. You can update the title, subtitle, and the icon. The icon is a little tricky and involves getting the filename. From the page icon chooser, you can right click on an icon there and “Open image in new tab”, then pull the filename, just the name not the file extension.
If you’re using this now, keep an eye out for the change and revisit these settings when they hit. There will be a grace period before it’s deprecated, but good to make the change when it pops up.