Because FGW will have to have HST power cars on the routes to the South West anyway.
So they can be any two power cars pulled from their pool of power cars for the Cornish services.
This tends to ameliorate the reliability issues that will supposedly afflict HST power cars.
The train manager could get one of those offices like those used on South West train services, which is a cubicle that uses rather less than 12 seats (and even if you only get 8 seats extra or somesuch, that is a significant improvement on the capacity of the seating vehicles in total)
Bar couplings to Buck eyes is not going to be an insanely expensive retrofit job especially since you only have to do a handful of vehicles that will always be used on the ends of the formation, (have a spare so if one breaks you can splice it in quickly).
And Mk3 loco hauled stock can easily be converted into HST trailers as they require no new equipment and it is merely a rewiring job, which they are probably going to need anyway as part of the refurbishment that they will need relatively soon.
And shortages of sleeping cars is irrelevent since what is desperately needed is more seated vehicles to enable that traffic to be expanded as it is rather easier to make money on than berth sleepers.
What is it about people on this forum trying to find expensive solutions for problems that just don't exist?
At the moment, no-one has actually decided what the long-term, or even medium-term train fleet for West Country services will be. Will someone be willing to foot the bill for converting HSTs with power doors, toilet tanks etc, etc? Or will they decide just buying more bi-mode IEPs will be less hassle? So there may not be any HST power cars anyway. In which case a modern diesel loco will be just what you need...
An office for a train manager sounds like a TGS, so that's all of one extra seating bay once you've smashed about a perfectly serviceable purpose-built BFO - a few extra seats are hardly going to make a huge difference to the bottom line frankly.
I suspect that rather than trying to turn loco-hauled coaches into an HST sleeper - and who would want a berth near a power car, you can feel every bump in a TGS or coach H on HSTs - passengers would probably prefer money to be spent on things like providing showers on the train.
As for your claim that what is needed are more seats, an article about this train in Rail last month gave me the impression that what is actually needed are more sleeper berths, with the indication that they could probably sell out six coaches going west on Friday nights and east on Sunday nights if they had the stock available. The Night Riviera already offers far more seats in the BFO and FO combined (though you only pay a standard class fare to use them) than the Scotrail sleeper portions. What makes you think there is demand for more seats?
hock: 
