OSMScout for Ubuntu is way outdated if you take into account the development of the library behind it - libosmscout. During this time, API has changed multiple times and the devs should be ready to work on these changes. Although, libosmscout community will help you via mailing list, if needed.
In general, we should join forces in developing apps for Ubuntu Touch, Sailfish (I am working on that platform), and Plasma. There are many similarities (Qt/QML/Linux) and we could surely benefit from working together.
As for offline vector maps, SFOS has progressed massively in the last few years. I am working on solution that provides offline/online OpenGL-accelerated maps, routing, search and spoken instructions. My solution was described here earlier at https://forums.ubports.com/topic/1022/offline-navigation-for-ubports/4 .
Recent development in terms of offline server: I have ported GUI to QtQuick. This, in my eyes, should help bringing it to UT as well, if I understand your architecture correctly.