Maybe this could help in some way https://discuss.flarum.org/d/27265-minecraft-auth
For store integration, my soon to be released Flamarkt could definitely be used as the marketplace framework.
I haven't heard of anyone writing an actual adapter between a MC server and Flarum yet. Even if someone did it I'm not sure why they would open-source it. It sounds specific enough to not have a big demand and also useful enough it would give a competitive advantage over other servers.
