There's not an easy solution - obviously there's going to be a conflict between bike capacity and seating capacity. The arrangements that seem to me to work best, other than having guards van/dvt etc, are what you see on the continent quite a bit - a generous all-purpose area where bikes can go but with fold down seats.
Where there's limited capacity in the system then obviously people who want to travel with bikes have to accept that there are limitations.
I just don't think, for practical reasons, that rotating handlebars is a workable solution. You'd need people to remember to show up with the right sized allan key, you'd need to have people check begore travelling that their headset wasn't seized up with rust as is often the case. And once you've rotated handlebars, bikes become a bit awkward to maneuver. It's regularly the case that you are ready with your bike but there's no indication of where the bike compartment will be, and/or when the train arrives one compartment is full of bags or prams and you then have to leg it down the platform to find another one. That would be very difficult with rotated handlebars. It's already a bit of a gamble as to whether you should have panniers off - if they are off it's easier to lift the bike in and maneuver it inside the train, but then you have to stick them on again if the abovementioned race down the platform is necessary. I doubt you'd find that most people who know what it's like travelling with bikes would be very enthusiastic about the idea of having an added operation to worry about.