We don't need Talia for that. We already saw Bruce implementing of the lessons that Henri Ducard/Ra's al Ghul taught him in both BATMAN BEGINS & THE DARK KNIGHT without needing Talia or Ra's al Ghul (in the latter). We already explored Bruce Wayne's past in BATMAN BEGINS.
I'm not sure what exactly you are saying.
Ok you're missing my first point
#1 I don't mean to actually go back in time to show us but, to have a physical representation of his past and the teachings of the LoS in the film not only will be a good structured point for the audience but, gives Bruce face to face confrontation with his teachings and how he's changed from Batman Begins. Not only that not being Ra's shows Bruce there's more out there, like him maybe even better. He's vulnerable, the LoS didn't care about the repercussions of their actions, that's what made them powerful. In BB Bats didn't care that much and in tDK his faced directly the repercussions of Batman;
Remember what Gordon said about escalation in BB and how Bats dismissed it? Well TDK is a product of that dismissal. Not only that his actions lead to the death of Rachel, the fall of Harvey and Joker's grip on the city, had he retained the LoS mindset, this wouldn't have affected him but, it has.
Talia will be a window into that world of losing Bruce and becoming only Batman, a window that he can stop being hurt and stop being the blame if he lets go that humanity in him.
Selena is the opposite, she fights for her humanity, revenge isn't beyond her and she's not concerned with who she has to hurt in order to do the greater good. She is Batman without limits; while Talia is Batman without a conscience.
That parallel is extremely important to how Bruce will react to his wanted status. He was strong enough to take the blame for Harvey but, is he strong enough to no longer be a crutch for Gotham and show the city that he can go "that" distance for the greater good? even if it means his total destruction?
Can Bruce Wayne learn to exist without the shadow of Batman hovering over him; as Rachel wished?
Edit: this is all ASSUMING Talia is in the film; I don't think this WILL happen but, if she was in it, I think this could be a possibility