I haven't read the thread, so sorry if I repeat/offend anyone.
I owned my first spaniel when I worked full-time.
From being 16 weeks old she would spend her days outside my office in the car, except of course in the height of summer when the days were hot.
I would go out and take her for a long walk at lunchtimes and after work (which was my main reason for buying her in the first place), but between 9:00 and 1:00, then 1:45 to 5:30-ish, she was in the hatchback of my Ford Escort. I always made sure she had water, in winter she had a hot-water bottle (refilled at lunchtime), on mildish days I would leave the back windows open a little.
She was a very happy sociable little dog, always perfectly willing, in fact eager, to get into the car (so she obviously didn't hate it).
I think that as long as you can guarantee a strategy for spending time with the dog, you are not parking in a dodgy area where the dog will be stolen (quite a risk with good pedigrees I believe), it really shouldn't present a problem at all.
Mind you, I do know someone who's Springer spaneil, completely ate the dashboard of his Mercedes, because it could smell polo mints left in the glove locker.