Edited the Heatran variable to be at 0 (not activated) instead of 1 (battleable) seems to do the trick to make the event no longer broken (or at least partially: Buck doesn't appear in the cutscene inside the mountain, but it doesn't cause any issue and he will appear normally for the cutscene outside of Stark Mountain, while Heatran will be able to be challenged after returning there).
POKEMON PL.sav