Ok, now that I see it I think the easiest fix is to slightly trim/piece the red to give you just a bit of skew and work the rest of the discrepancy out/in by tugging. Even a slight diagonal adds inches over a long distance.
The hard thing is you already have the other sides on -- and they look lovely. To do it with the points touching is a master move and I'm not surprised that some issue crept in. Is much easier to do "floating" with the points not touching and coping strips.
Part of the idea of the coping strips is not just going from a square to diagonal set as I mentioned before, but also to square up projects. Like in a t-shirt quilt or a sampler where all the blocks aren't the same size, what you do is frame them to the correct size and while doing that you can straighten them out.
You may not be enjoying this project much right now but step back, take a deep breath and acknowledge that it is a beautiful piece. Because it is
edit: Ooo ooo ooo, RhondaK has a wonderful idea, can you make just a couple of the link sections a hair bigger/smaller and just replace maybe 3-6 units of the border that way? That would be pretty sweet and that's why we share our ideas!