Actually, you're spot-on. The issues I referred to above would only have occurred with 8 coaches + generator van.
There was originally a full spare Enterprise coach rake, but as demand grew they plucked coaches from that set and added them to the others, meaning there was only 4(?) spares, including the DVT.
The current arrangement will make it easier to rotate coaches for refurbishment when the time comes (i.e. in the not-too-distant future).