A slightly different thought. I always use roomettes so I'm always parallel. The question is head first or feet first. I prefer feet first as a sudden stop won't crunch my head. I can't remember if the attendants set you up this way. I always sit during the day facing forward.
While I agree with your assessment for sleeping, although as MrFSS pointed out that's not always possible to do, when it comes to sitting during the day you've got it backwards. If you're concerned about being safe in a sudden stop, then you want to sit facing backwards. That way in a sudden stop the chair cushions your body and keeps you from slamming forward. The chair will support your spine and neck, as well as your head.
If you're facing forward, you're going to fly across the compartment and into the other seat. Of if you have your legs out and brace yourself; there is at least a chance that you could end up with broken legs.
Granted all of that is very worst case scenario, as the train pretty much has to hit a brick wall to stop that abruptly.