Thanks for the follow-up!
I'm glad we have a more helpful error message now. We haven't scheduled pulldata implementation yet but we're getting very close to having all the pieces in place for it so it may not be long. I don't think we'll make it possible to attach files for the preview, though, so we'll likely need to try forms with attachments on Central.
The XLSForm has a lot of unnamed groups and an unnamed repeat.
That's not a problem. I often deliberately omit group labels for stylistic reasons so I have wondered whether we should consider dialing back these warnings. Repeat labels are very useful for Collect, though!