Having read that article you posted where they speak to Cathro, I'm wondering whether part of him agreeing to come here is that if he sticks it out until McClaren goes, he gets the gig.
Even makes me think that the strange way they said "three-year deal, possibly rising to eight" about McClaren's contract is that they always had a young, progressive assistant in mind (Neville, Zenden, Cathro); they evaluate whether McClaren might move upstairs permanently to assist/replace Carr after the three years and if so, ready-made replacement who already knows the club, city, squad etc.
Of course, that all sounds far too organised and forward-planned for our bloody cowboy outfit