I would say all your observations are pretty accurate, dasBlog is a mature if not perfect product, and the core dev team have other interests, so the project is in low maintenance mode (my opinion).
But I will try to address some of the issues, while not recommending you stay or leave dasBlog.
Hosting dasBlog in a subdirectory is doable as long as the directory is a IIS Virtual root.
We don't really have an image gallery option, some image support but no image gallery, not even sure I know what you mean by image gallery?
Agreed many of the features and their are tons, are not well documented, all over the net, in the code, on the authors site etc.
Agreed on documentation.
User-interface is a project of thirty plus authors in a very disjointed process over many years, this app is the grand daddy of c# web apps and probably a poster child of how not do it.
User forum shows the public mind share has shifted to other products, agreed.
Link to documentation is what you get, that’s it,
DasBlog.net has never really belonged to the project, the original company that sponsored the initial version in Germany had no interest in continuing and let it go back to the public, we tried get it without luck.
So your impression is about right, we now only have two or three people working sporadically on dasBlog, your call if you join or not.