Personally I have never been a fan of the idea that kata techniques keep things hidden. The goal of training, any training including kata, is to improve technique, build muscle memory, and optimize timing in order to improve your chances of a better outcome when you need it. Having the kata move you in ways opposite of those ultimate goals are not going to accomplish any of that as you will have to think too much about how to do it correctly in an already stressful situation. So the premise that when going backwards in the kata you would really go forward in a fight makes no sense to me. If your strategy is to always be going forward in a fight, then that is how you should train.
I can think of no other comparable activity (any sport for example) that has you learn a component of it by repeatedly doing it in reverse, or any other way then how the trainers want you to do it when you need it. In my mind, if any kata really is doing that then it should be dropped from the curriculum. But then again, I am biased towards what works for me, maybe some can make that whole going forwards backwards idea work.
At any rate, I do not see anything in the kata as symbolic, it all has function. Or at least it should. To me a more important consideration is whether Seisan even originally had a jump in that sequence, that little detail can definitely change the functional interpretation of its application.
Likewise, I doubt the combatants of old who came up with this stuff really took anything as given with their training, including movement and avoidance. Too much was riding on it and they would have worked to perfect everything in their toolkit...well, the successful ones at least!
Regarding avoidance, if done correctly getting an opponent to miss his target can set you up in a good position to inflict some damage before he can recover, so I would not discount it too quickly. Personally that is my interpretation of the jumpback and following forward attack sequence that ends Seisan, so I prefer to not jump very far back so as to be in a better attack position, as opposed to some I have known who will place a stick or other item a ways back on the floor to jump over with the intent of maximizing how far back they jump and then how far forward they jump in to attack.
No disrespect intended to anyone who trains that way, or with anything else I have said above, there is plenty of room for differences in interpretation and opinion. Mine are just where I am at the moment.