Hi Pizza90
You have a lot of good questions, and I see you've done your homework.

I started playing MG in December of 2015 and became a contributor of maps and bug reports, and a little $, and some very minor patches. The developers of MegaGlest have no affiliation with ZetaGlest or the new Glest project. ZetaGlest was a project me and Jammy founded as an experiment basically, to see what we would be able to do with the code (there were some features we were interested in implementing that aren't part of MegaGlest).
While maintaining ZetaGlest, a dev named Mathusummut joined and became the primary developer and Windows expert. After a year of working on ZG, I turned it over to him, as I didn't have a strong desire to continue leading a project of that type and scope for an indefinite period of time (most likely years, lol).
He recently changed the project name to Glest. I have no idea why that name was chosen but I agree with you about it being confusing.
After that change, I noticed that most ZetaGlest pages were leading to 404 errors, so I recreated the org on GitHub, and re-added some related repos to archive them and added a
blog post trying to explain the current status.
I went to back to playing and enjoying MG on a semi-regular basis, and am in regular contact with titi so I can probably answer the rest of your questions until you hear from the MG team directly.
As for MegaGlest, there have been many many commits since the release of 3.13.0, although there hasn't much activity lately. As you noted, a commit a few days ago, but before that there were several around July of last year. There is no date scheduled for the next release but afaik they have no plans at the moment to deprecate the project.
Any chance to implement recent modifications they added?
We can't expect the MG devs to have the time to stay completely informed about new features of Glest, so that's too vague a question (though a good one!). Individual features and requests are still discussed on the forum here at
https://forum.megaglest.org/index.php?board=21.0 Though obviously sometimes the discussions aren't engaged in at a swift pace.

Is there a roadmap to look at for undrstanding what will be implemented in the future?
None that I'm aware of.
If there's any other details about ZetaGlest you'd like, feel free to
contact me directly, forum PM, or use the Off-topic section of the MG forum.