@karry, great questions! While I cannot comment on Silica license, few comments regarding Kirigami/UT.
I think it makes more sense to use native platform UI and not try to push Silica into Plasma mobile or Ubuntu Touch. For Pure Maps and OSM Scout Server, I wrote QML wrappers that allow me to support multiple platforms with the same QML code in the app. It is not something that would perfectly fit you immediately, but could be a reasonable starting point. @piggz is porting his app to it, so maybe he can chip in and tell his experience. in particular, what are the issues that he has seen.
When comparing to the use of Silica on other platforms, switch to such wrapper takes time of the developer. I just feel that the result is worth it.