The layer name sorting bug will be fixed in Rhino 6.2 which is scheduled to ship in about 5 weeks. The fix is that layer names in Rhino 6 will sort exactly like they did in Rhino 5 on US-English locales, and probably most other locales. In particular, all common symbols, including underscore (_) will sort before numerals and latin letter glyphs.
Details are in https://mcneel.myjetbrains.com/youtrack/issue/RH-41224