Breathing....well they breath like normal human beings.
it isn't that hard...I race and it's natural the helmet doesn't influence you in any way.
The 2 horns work like this....the high speed air moving on top of the helmet creates a lower pressure system on top of the helmet....the air in side the helmet is at a stand-still (low speed high pressure) so the pressure gradient will suck the air from inside the helmet. There are small chanels inside the helmet the help to cool down the drivers head....
What plastic thing on mikas helmet...? I can't really remember! could you post a pic?