So is there a rule to pulling this shit on regular basis or not?
I've said before Avalon need to research whatever advance portal technology there is to be able to transport adventurers directly to the entrances of all the other demon lords dungeon so he can have the humans do the raiding for him while he heals and buffs and build weapons for them for free to ensure their survival while pushing other demon lords to the brink without having to use any of his own units and not breaking any rules.
And with portals to everywhere from Avalon it will be the de facto center of the universe because it just makes sense for everyone to live there for the traveling convenience.
Even if he gets discovered as a demon lord by that point what are the people going to do? Kill him and set their own standards of living back 1000 years? Yeah that makes sense.
Several of the influential humans know what he is, and are ok with that as its beneficial for everyone.
As for teleporting, its really not so simple, to teleport you need a monster that has a certain skill to set warp points at both sides, they can be destroyed and used by any other monster with that skill. Only the monsters with the skill can use the portals, but can take a limited number with them when teleporting. B rank monsters are the lowest able to have the skill, and they cant even take more than 5 people per teleport and can only teleport a few times a day. You would basically need a monster per group you are teleporting, and the portals could only go to friendly areas as they would just be disabled otherwise. Even if he summons a S rank it would need to travel with every group teleporting.
Sending adventurers would be fine, and he can gear them even. But sending them by teleportation just isnt practical. He also is in fact allowed to attack them directly with his monsters, the protection is that old demon lords cant target new demon lords, He is allowed to pick a fight, however if he does then he is no longer off limits to the one he attacks.
Now I'm really not clear why this very blatant direct attack was allowed. I assume it was another new demon lord, as otherwise they protection should have activated, and the creator god may play around a lot, but you play by his rules.