Step # 1
If your cats allergies are that severe we really need to figure out whats causing the problem and to soothe the inflammation in the meantime. This will have to be done with your DVM.
Step # 2
We don't put plastic e-collars on our cats instead we have a softer solution - we use EZ Soft E-collars, they are made with fabric so the cat doesn't feel the impact of banging into walls. It is not dangerous to wear these.
I would also trim your cats nails to reduce the damage she does to herself, keep her indoors as well to reduce risk of infection from open wounds and to keep her safe (less defense).
Note: E-collars are for temporary protection and are not to be substituted for treatment.