Update - I got the hanger with the derailleur off the frame today. The seized bolt is not seized in the hanger! It rotates freely in the hanger, in fact that is how the RD manages to pivot. But the bolt does not rotate inside the barrel, through which it passes (and which must contain a spring). On my other two bikes this bolt does rotate inside the barrel.

The question I have is - can this bolt even be replaced? I looked it up in the Shimano docs, and they list part number Y5Y498010 - B-axle assembly. Has anyone replaced this B-axle assembly and is it worth it?