The two criteria above must be met, and both:
1
TOWN_INFERNO
The number of gate openings can be easily adjusted with this parameter. If it's two, it can open twice.
Just to be sure, I checked ABILITY_DEMONIC; its absence won't prevent a creature from opening a gate if the town type is Inferno and SpellPoints2 is available. (but it is important for Holy Word and Fire Wave)
Maybe you'll upgrade your creatures that are neutral and demons at the same time?))))
I'll also check if an 8th-level Incubus under the Amulet of Mastery will open a gate for a Demon.
They can. If you create demonic neutrals correctly, with the right race and spell points, they will be able to do what a Demon needs to do, even at level 8 with the Amulet.
Including Balgors and Antichrists
They will still differ slightly in their description - for Nunukis, when opening a creature, the town is not "Unknown," but Inferno. For Incubuses, it's "Unknown." I think it's because the creature is level 8 and has the "Unknown" tag in its description. As it should, gating works for Incubuses with the Amulet of Mastery (4th gating, doesn't work without the Amulet, as it should) and the correct creature file description.