Hello Lily
I think you should consider your corset an aid - I'm sure the DWP will - but when you mention it in your claim you MUST explain how wearing it affects you. Keep a diary of how often you wear it , and how you feel after you have been wearing it for any length of time. I think you need to show that you wear it for the majority of time in order to be able to function at any level to aid mobility, but that if you do wear it, it is uncomfortable.
Is it uncomfortable to the extent you are in pain when wearing it, because if this is the case you must mention that also. Do you need to take pain relief when you are wearing it, or rather do you need to take MORE pain relief than you would if you weren't wearing it?
The thing to remember is that PIP refers to how your disability affects you, and if your corset and/or medication are not giving you the help and relief you need then you must stress this.
Do you need help to get into the corset? If so, explain this in your form.
I think that's all you can do; so long as you explain everything fully. you will give yourself the best chance of success. Good luck with your claim