Those are implementation details internal to namei.c.  Certainly this
function wouldn't use all of the fields in nameidata, but it doesn't
hurt to have a few fields in a struct on the stack which don't get used.
Keeping the code simple and uniform is much more important.  Using

Exactly.

Certainly vfs_walk_ancestors() would fallback to ref-walk if rcu-walk
resulted in -ECHILD - just like all other path walking code in namei.c.
This would be largely transparent to the caller - the caller would only
see that the callback received a NULL path indicating a restart.  It
wouldn't need to know why.

Yes, that's also what I mentioned in an earlier mail.