Oh duh. I just read through the thread again, and you said you already tried to lengthen them. Sorry for suggesting what you already tried. Still... I can't help but think your stirrup-length almost looks like you're ready to go jumping. You said you lose your iron at a canter with a longer stirrup. You shouldn't. I wonder, at a canter, do you clench on with your legs almost as if you're riding bareback? That can bring your leg and heel up and out of the stirrup. Relax at the canter, sit with your back tall and upright and your butt deep into the seat. Let your butt and thighs just melt into the saddle, and make your legs molten lead, flowing into your heels. Don't clamp on.
I love your position when you're up in the posting trot. Many people really lose their leg at that point, and you just have a really nice heel underneath you and a nice up[right posture. Very nice. You are quite far out of the saddle on the "up" of the post, which again makes me think that your stirrups really are too short.