I mostly agree with your suggestions. As I mentioned on one of the Talk pages I would also explicitly tag unnamed guideposts with noname=yes to distinguish them from those where the name just hasn't been added yet.
However, I don't see any need for a requirement for each way to be only in a single base network relation. It seems better to map all relations up to a named guidepost, especially because that's how the base network routes are signposted. The only potential issue (the one already mentioned:
https://github.com/waymarkedtrails/waymarked-trails-site/issues/312) is when (base network) routes with different difficulty share one section. The signage must already mark them as a (white-red-white) mountain hiking trail even if an initial section is shared with a (yellow) hiking trail, but a renderer might want to show only the easiest base network marking available for each section.