Okay, few incorrect statements here....
Firstly, demonic influence would always spread to your borders, peace or no; You're Infernal. Your entire empire is Hell. Demons spawn on ANY hell tile, regardless of whether or not it is controlled.
Inability to fight demons for experience makes perfect sense; Instead, you should be spreading hell and trying to get them to terrorize your enemies. They are, for the most part, simply an autonomous part of your empire.
The demonic lair is actually a valid complaint, though I'm not sure anything will be done; More pressing issues first.
Animals can cross the border only after peace with demons? No, they can always do this.
HN is not in Hyborem's style. Nor, IMO, Bhall's, who would at this point be the principal leader; Agares mostly sits on his thrown and wallows in despair. Chaotic implies they will do whatever they want; It does not imply, not does it guarantee, that they would do so in a sneaky way. Put it this way: HN fighters would tend to be Chaotic, but Chaotic does not tend to be HN.
In short: Sure, them killing off your HN units can be annoying, and I can see why you think it's an issue. But it is a VERY minor one, that is IMO more than made up for by the benefits of peace, and one shared by all other civs who are at peace with barbarians. If anything is done, it would be well in the future.