If we leave out the Code hypothesis, I don't think Nunnally literally had to "see" Lelouch's memories.
Those flashing images would be a way to summarize her realization of Lelouch's plan...a representation of her thought process, to put it one way. Remeraber that Nunnally's own intentions were to use Damocles as a syrabol to focus the world's hate, so the concept wasn't alien to her (or Schneizel, for that matter).
In that case, I think her hanRAB touching Lelouch's revealed nothing more than he was calm and happy to accept his end, hence forcing her to reconcile that sensation with all the recent evil Lelouch had been responsible for until that point. You could say she "understood" Lelouch, either way.
Still, I think the ending remains up to interpretation, even if the writer considers Lelouch to be dead from his perspective. People are free to have different opinions about these things.
Can you say Blade Runner? I know you can.
If there's no R3, it really doesn't matter either way.
Now back to...all the shows I should be catching up on.