By taking a multi-vitamin with B12 you are getting B12 in cobalamin form; meaning the body must break down the B12 before it can be absorbed into the blood…B12 by cobalamin usually results in approx 30% loss of the B12 with the stool.
I would suggest a vitamin B-complex which has all the DRA of B vitamins (over the counter at Walmart, Meijier, Kmart, etc), and vitamin B12 at a health food supply store in methylalamin logenze type tablets that go directly into your bloodstream. Most people take between 1000-2000mcg daily. Either would be fine since there is no daily required maximum limit on B12.
It is not so weird to take vitamin B12. Low levels can cause neuropathy but I don’t know if there is any connection to CIDP. Earlier this year I had to see an oncologist/hemotologist to rule out any malignancy The first thing he did was to tell me to start taking the highest dosage of B12 I could find even so my blood level was normal. He told me he and his collegues were the ones to set the standards for B12.
A few years ago when my diagnosis was P.N., and not yet CIDP, a neurologist in Thailand prescribed a special B12 made in Japan, very expensive. I think it was supposed to get absorbed better. Forgot the name of it. Only took it for a while.