the electric car for a Voyager has been discounted as being far too expensive
Last that was heard (in 2011) was that the DfT were "concerned" about the costs. The fact that nothing's been heard since then is a good indication that other things were considered a high priority, but since we've not had any new stock or franchise changes on XC (or MML or WCML, also users of 22x stock) means there's not really been any opportunity for Bombardier/the ROSCOs to actively "pitch" the idea to the operators. Indeed, even if the costs are high, if the alternative is that the fleets sit idle after being replaced by 80x bi-modes, the ROSCOs might well take on the option out of their own pockets.
One usenet post from 2012 (can't really link it, since Google Groups resists direct linking), citing "Informed Sources" states that the eVoyager programme was cancelled partly due to "the expanding electrification programme means that it will be possible to cover more Voyager-type duties with straight EMUs." Since this is now not happening, that changes the business case.
It also states that "In its ICWC bid Virgin proposed replacing Voyagers with Baby Pendolinos.", something that is also pretty unlikely now.
As it would be a new build it need a latest emission regs diesel engine which means the fleet would not be uniform- different spares and maintenance regime for that one vehicle in the rake.
As I understand it, the extra vehicle woudn't have a diesel engine (or possibly even traction motors), it would be a pure trailer. It's not as though 22x units lack power; it's fairly common for 222s to run with one engine down without major effect on timings.