The neutral is sized to 140% of the unbalanced load.
You are describing a bolted fault in the feeder itself (the sub-panel O/C device protects the branch circuits) and if the code addressed that you could never put small conductors in a pipe with larger ones.
You are right that the EGC must be sized by 250.122 but I don't think that applies to the neutral if the computed unbalanced load is significantly smaller than the total load.
I am certainly open to be shown wrong tho.
