@MartinK: if you have issues, check which repository was used at mine OBS project. Some dependencies were built, at least at some point, against the patched versions. It may have been to resolve SFOS case, but maybe something else.

prime_server is ZMQ-based server used in Valhalla and is needed, indeed.

As for OSM Scout Server itself, you may want to look at PRO file and suggest a better way to specify valhalla's and fonts path. Since you have more experience with packaging, I would wait for your advice on it.