All my extensions got published today. \o/
It seems one of the main issues was that the build scripts did not run on Windows, but I was not contacted about the error, and with time the extensions were likely pushed down in the reviewer backlog.
The submission experience could be improved by communicating issues as they happen, and the publishing guidelines also need an update.
The guidelines currently suggest that bundled code is not accepted. That is no longer the case, we just have to provide the source code and instructions to reproduce the submitted package on Windows.
https://dev.opera.com/extensions/publishing-guidelines/#acceptance-criteria
If there are others with bundled extensions, this is the guide I've sent them that finally got the exensions published, feel free to adapt it to your own needs:
https://github.com/dessant/search-by-image/wiki/Building-the-extension-on-Windows