Many times we see the Icelandic Horses fighting the bit. It would appear that the jointed bits bother the interior of the Icelandic Horse's mouth. The heavy contact compounds that problem for the horse.
The horse tries to open his mouth to get away from the action of the bit and the harsh contact.
The noseband is tightened up to keep the horse's mouth closed on the bit.
Is that the answer?
Many horses in the world do not wear nosebands and do not open their mouths to get away from the bit.
If something is wrong, we need to find out what it is. If the noseband hides the problem, the problem is still there. A good horseman will not let the horse suffer if he is trying to tell us something is wrong.
Let's start with tossing out the nosebands, and correcting the problems caused by the bits and the heavy contact.
If the jointed bits are a problem for the conformation of the interior of the mouth of the Icelandic Horse, let's use a solid mouthpiece, perhaps a mullen-mouth bit, no shanks.
If the heavy contact is a problem for the horse, let's try riding on a loose or casual rein.
The Icelandic Horse is a smart horse. He does not need or want to be micro-managed through his mouth, while being ridden.
With a smart horse, a well-trained horse, and a good rider, the horse doesn't even need to wear a bit; he can go bitless.
Some other articles to read about Icelandic Horses and bits: