However, even after installing 3.15 I had no luck first, UNTIL I followed the advice in this forum to delete my "Helix Native" folder in /Library/Application Support/Line 6 (do only if you have backups etc., you will lose all settings, IRs etc). Probably some of the old settings killed the plugin. Maybe I should have tried that even prior V3.15, and Helix Native might have worked under Rosetta somehow, however Logic runs now natively and Helixe Native works fine.
Line6 Helix Native 3.15