Aircraft seats are designed to be fairly flimsy and move with passengers in an incident. Again, this argument come back to cushioning. The design of seat frame is not an issue, it is how the cushion and foam are designed that sits in that frame.
Should be noted that those look like the original 1991 seats, albeit with the plastic recoated and the covers replaced. I wouldn't be surprised if the actual cushions date from 1999 when Northern Spirit replaced them. Should remember that one of the primary reasons for thin cushions is usually weight - which equals lower energy / fuel costs ...or insurance - mitigating fire risk equals lower premium costs.
The basic standard for aircraft seats is surviving a force up to 16G during an impact. Not flimsy at all.
Comfortable train seats are perfectly possible, as shown by the GWR refurbishment, here's a 158: https://www.flickr.com/photos/camperdown/43359050284
According to a designer friend who's worked on various prominent train classes - yes! But that's no excuse for the uncomfortable seats. That's what I'm getting at.
The deeper cushions fitted by Northern on 158752-158759 were indeed an abomination as they were too firm and too deep, and, as you say, meant you sat too high. The design used by other operators was more forgiving and not quite so deep, improving the comfort relative to the original seat cushions, which were prone to collapse.
I'll take those seats over an IET any day. I travel Bristol-cardiff for work regularly and try and get the Portsmouth trains if I can.
The thicker cushions in the centre cars were in fact fitted by Northern Spirit back in the late 90s (obviously the cushions will have been replaced a few times since then, but that was the first attempt at making 158s somewhat suitable for a quasi-intercity service). In my opinion they're the comfiest standard class seats on the railway by a country mile. You're entitled to your own opinion but "abomination" is just a tad hyperbolic, don't you think?
Many people (including myself) get travel sick, and I find this is particularly the case on a coach. Throw in lack of legroom, flimsy tray tables that barely support a coffee (and I find totally unsuitable for a laptop), chemical toilets (if even available) and lack of opportunity to stretch your legs. A coach also tends to be slower and doesn't (to the best of my knowledge) offer delay repay.
Not going to hugely disagree BUT everyone using the toilet on a coach should sit down. Part of the reason they're often less than clean is men determined to stay standing. Some operators have signs saying please sit, maybe all of them, I don't know.
Not sure if it is true, but I found the seats of the LNER Class 801 to be more comfortable than those of the GWR Class 800/802.
I have travelled quite a bit on both and find that individual seats vary quite a lot. I spent one journey from King's Cross to York with a screw very noticeable in the seat poking me. All IET seats I have had have been less comfortable than the NatEx coach although I will mention I've used NatEx much less than the trains.
I had the same on LNER last weekend. Could feel the metal.
