Oh my goodness ... you need to find some better trained divers to dive with. Most of what you just put out there as "can't do" are things I do regularly.
- A sidemount diver does not need any more bench space than someone in backmounted doubles. If they're taking up more than that, then they need to acquire better techniques (or just give a crap) for managing their gear on a boat.
If the boat is really rocking and everybody is gearing up in place, at their bench, don't you take up a lot more width on the bench with SM doubles?
If circumstances dictate that you gear up in place then walk to the exit and get in, won't you need a lot more width to walk from your bench to the exit? Which, if the boat isn't setup for that extra width, could mean banging tanks into other people's knees, etc., as you try to get by to get to the exit?
Of course, you can say "yes, so I wait until the people on either side of me have finished gearing up and headed for the exit before I attach my tanks" and "I wait until everyone between me and the exit has already headed for the exit". But, if you need to put those restrictions on yourself, that still seems like a drawback to using SM doubles on some boats.