I find it interesting that the iron deal with Isabella, a vital part of the war vs. HC/Izzy deal has yet to be mentioned. Right now, you are sending iron over to her, which is undoubtedly fueling her forces against Huayna. Furthermore, a quick look through her lands reveals that she only has a copper right on her border with HC, which we can assume will be pillaged or in some way disrupted.
If you declare on Izzy, she will lose the iron, and also being (most likely) without copper, you are pretty much signing her death warrant. BUT, who will reap the rewards? Not you. HC will take everything and put himself in a great position, especially by only having one foreign border (which is with you).
If you declare on HC, the iron deal will be cancelled because you can't have any trade routes through his land. This one is a bit more tricky though, because we can once again assume that Izzy will be on the losing end. However, HC will be diverting half of his forces to the Western front with Izzy, so with a decent army you should be able to do him some damage. It might also be beneficial that Izzy not be too successful against him for fear that he might vassalize to her or she would just become more powerful with the land she might take.
The last option is to do nothing. It's hard to be neutral because we can assume (especially if you plan on adopting Hinduism as you or someone else mentioned) that one of them will ask you to declare or at least HC will ask that you cancel deals with Izzy. Then you arrive at one of the positions above and will have to make a difficult decision.
I am not taking a side here because I have not followed the game very well and am not fully knowledgable of the situation, however I just wanted to point this whole thing out. Good luck in the rounds to come.