Given the difficulty of lengthening the shell, I decided to go with the longer size, a 25. The heel was loose, so I inserted a Dalbello heel cradle. The boot is not a Dalbello, but the cradle seems to work well---my heel is not lifting, nor moving laterally.
So now what I have is a boot that's a bit too long, but my foot is held fairly securely all around. Is there anything wrong with that?
I know the general recommendation is to go with the shorter size, but that has gotten me into real trouble in the past.