I've not read through the whole thread, but given that I travelled on the route last month, I thought I would give my opinion.
I travelled from Sheffield to Liverpool Lime Street and return and think that the 158's are ideal for the route- I found the seats comfortable, there were plenty of table seats, there is a toilet in every carriage and there is air conditioning (although I am aware it isn't very reliable).
In fact, I posted in another thread some time ago that I think DMU's are unsuitable for long distance runs. After my experience last month, I take that back as I found the train surprisingly smooth and quiet for a DMU. I thought the unit I was on during the outward journey was slightly quieter than the return (the engine seemed to throb ([s that the right word? It vibrate loudly, and then went quiet, and then went louder again in a cycle] when decelerating, and there was a revving sound present when stationary at lower speeds- could it be the air conditioning?), but overall, I thought they are good units. The only thing that could be improved in my opinion is the PA system as it was ridiculously quiet in both directions. Also, a small buffet counter would be nice as the advertised trolley service didn't show up during the outward journey.