My understanding is different; that if a doctor accepts Medicare assignment, then if Medicare doesn’t approve a procedure, the doctor doesn’t get paid for it.
I may be mistaken, but that’s what I’ve read.
The burden is on the doctor’s office to properly document the reasons for treatment.
I think your injections are covered. But likely under part D rather than B for convoluted and confusing reasons. So you might want to check your drug plan formulary, because with D there is likely some sort of copay.