I've tried several times, but your message is all over the place. I'm trying to look into this, but in the future, it'd be better to clearly state what each realm's diplomatic relations to the other realms are, the battle region (which you did, thanks) and region owner.
Generally, if you want to support someone in a battle, you need to be allied or federated. If you want to war someone, you need to be at war with someone. If Keplerstan is at war with Evilstani and at peace with Goodstani who is also at war with Evilstani, and Evilstani is attacking Goodstani's region, Keplerstan moving into help defend Goodstani's region will not work, as I understand it. You must have an alliance.
Now the confusing sections...
There's no diplomatic reason realms at peace wouldn't join in an attack against a realm that they're both at War with because of a pre-existing alliance.
It seems you are referring to them as both at-peace and allied.
Again, its two realms in alliance, and one at peace with both of those, but all three are at War with the other
This was the most nonsensical. You refer to three realms, Highmarch, Vix Tiramora, Red Haven. You say two are allied, and one is at peace with both of those, but all three are at war? How can they be allied and at peace and at war with each other?
The whole point of us being there is to actually fight alongside Highmarch.
Then you need to be allied with them.
Is there anything I can do to help this fix along or are we just going to have to go to Alliance status with Highmarch for game reasons and pretend we're still at Peace Diplomatically?
There is no pretending. If the game says you are allied, then you are allied, not at peace. If the game says you are at peace, then you are at peace, not allied. No different than the placeholder rule (if you are elected/game says you hold a position, you hold that position, not temporarily holding it for someone else to return). If you were only at peace, not allied, then the only reason you were not fighting together is that you were not allies, which is your diplomacy decisions, not game code. You don't risk life and limb to fight with folks you are merely peaceful with against a mutual enemy, you let them weaken each other first.
