That’s accurate. The level of support Mozilla gives Thunderbird has varied along the years and traditionally hasn’t been great. All add-on documentation and support is handled by a small group of contributors who work really hard on keeping the project alive. I don’t expect this situation to improve in the future, to be frank.

For Firefox, XUL extensions are planned to be dropped by the end of 2017, and dropping XUL entirely is just a proposal that could take years to complete. For Thunderbird such changes might never happen. To drop XUL extensions there would need to be support for the new API (WebExtensions) in Thunderbird, and I know of no plans to do this.

As far as I know, extension signing won’t be a requirement for Thunderbird.

I suggest you give the Thunderbird planning list a look. You may find the answers to your other questions, or get answers directly from the people in charge.