Given past enmity between the two, is there not at least a chance that he went along with the campaign on the basis that Blair would come out of it looking shabbier than when he went in?

Brown isn't that devious. He doesn't do calculated.

I think it's more likely someone suggested Ashton to him, and when it was obvious that the other fellow travellers were out, he jumped at the chance to shoo-in someone who could wave the flag.

I wouldn't expect plots within plots from Brown. He's neither adroit nor imaginative.

As for why - yes indeed, it was to keep the UK on side. Not picking a Brit for the big jobs would have given the Eurosceptic gutter press in the UK its best Christmas ever.