Is there any doubt the DFT could bring improvements if they wanted to? They oversee everything. They could request XC to begin a tender for additional stock, just as the DFT I believe requested East Midlands Trains to start a tendering process for fitting retention tanks and other basic PRM mods to the HST fleet regardless that they were the outgoing TOC.
Stock could be hired in if the situation was deemed necessary surely? Just as it's been hired in for ScotRail Fife Circle' and the recent Norwich Great Yarmouth/Lowestoft Anglia service.
DfT could specify improvements. But those inprovements will come with a price tag attached, essentially stipulated by Arriva.
