1. gray sort values are just empty sort values; they are duplicates of corresponding "not-sorted values". these original "not-sorted values" will be used for actual sorting if "sort values" are empty.
2. i don't use "sort tags" in MB, so I can't help much, but MB has two independent "sorting approaches." one uses physical tags, and another (more powerful) uses an MB database only. maybe other forum members could explain it in more detail. or you can search the forum for MB sorting options.
3. i don't think that anything except a semicolon will be accepted by MB as a multi-value separator.