[general remark there are a lot more people present on the talk-ch mailing list and it would make more sense to pose any questiosn there]
The easiest way (without using a specific API and for routes only) is simply to check with https://hiking.waymarkedtrails.org/#route?id=5635840&map=15!46.9237!8.451
The other reasonably easy method is to check with http://overpass-turbo.osm.ch/
It is the nature of the beast that you can only find relations in areas where they have members, so it will always be difficult if you are starting at the wrong end of one :-).
Sure, but no idea how to do it with iD, with JOSM it is simple.
The osmc tag is really an oddity and not that important, what colour are the signs in real life (because that is the actual criteria)?
Simon