You’re relying on a false dichotomy, that Sakura can’t have any romantic feelings for Naruto if she still has romantic feelings for Sasuke.
There is plenty of proof for the argument that Sakura has been developing romantic feelings for Naruto over part two, without her realizing the nature of those feelings, while still continuing to consciously cling to her romantic feelings for Sasuke.
Her blatant attraction to Naruto in 245, Yamato’s words to her in 297 and her reaction to them, the fact that she was bothered that Hinata loves Naruto in 442, her hugging him in 450 (and the crowds reaction to it), and the clear distinction in her lying abilities when she was talking about her feeling for Sasuke and her feelings for Naruto in her “painful” confession at the summit all support it.
The third data book even outright states that she doesn’t know what she feels for Naruto currently, but that he makes her heart beat faster.
Why didn’t Sakura come to a love epiphany when Sai told her Naruto loves her, and during her confession at the summit? Because it wasn’t the proper time for it. Sakura’s character wasn’t where she needed to be in order to realize her own feelings. She needed that second encounter with Sasuke to shake her view of him, and the words of an outsider with no knowledge of the Team 7 love triangle to get her thinking differently about her feelings for Sasuke.
540 makes more sense as the catalyst to consciously being the change of Sakura’s feelings for Sasuke, rather than a pointless (and rather ludicrous) reminder of her feelings for Sasuke still being there when we were reminded of it just last arc.
Will a love epiphany on Sakura’s part be cliché? Of course. But is it unsupported? No.
First of all, Naruto confessing is primarily about Naruto's character, and properly resolving his romantic feelings for her. If Hinata and Sakura got the chance to confess and directly express their feelings, logic dictates Naruto, as the main character, will get the same chance whether or not it leads to NS canonization.
Secondly, its not about being a hero at all, its about Sakura coming to realizations about herself and her feelings. Naruto confirming that he loves her (since she clearly doubts his romantic feelings for her at the summit) and why he loves her (since Sai gave no reasons), will only help Sakura's character (since again, she seems to be of the belief that she's not worthy of Naruto's love - hence her words in 459 that he's an idiot for loving her) in regards to her confidence issues and reinforce Yamato's words (that her caring for Naruto as much as she does is enough).
And honestly, with the amount of talk-no-jutsuing that goes on in this manga from Naruto, you're doubting his ability to change people's hearts and way of thinking with his words alone?
I'm not talking about the subjective merits of this happening, but its likelihood given what Kishi has shown us in his manga thus far.
Her feelings for Sasuke are far from immutable. She was willing to drop the search for Sasuke after VOTE in part one after Naruto first failed to bring him back, and gave up on his redemption at the summit as well. She's encountered Sasuke twice in the three to four years after her confession, and the second encounter is enough to get her reflecting unhappily on her romantic feelings for him.
She's been clinging onto a memory of Sasuke for all these years, even after the first encounter. Of course she's going to have difficulty reconciling the Sasuke she knew, the image of him she's built up in her head, to the vastly different one she's hearing about from Karui. Hearing about the things Sasuke has done is one thing, but seeing for yourself in person is another thing entirely.
When hearing that the rookies had decided Sasuke needed to die, who was the only person we saw her focusing on? Naruto, and his emotional pain - not Sasuke and his impending death. We know from the summit she eventually thought about it, but Naruto was clearly her priority. I think that's pretty telling in regards to the respective strength of her feelings for the boys.
She is the only one out of the four players in the romance subplot to have a current negative view of her love interest, so logically, if anyone is going to change their feelings at this point in the manga, she seems to be in the best position for it (especially given that she is also the only one to show anything beyond friendship for the person who has unrequited feelings for her).
No one is bragging here. You think the reasoning for NaruSaku canon is weak. You are entitled to you own opinion, but in the end, that's all it is - an opinion.
I'm not sure how can you say Sakura's standards are high at all - she's in love with Sasuke, someone who she thought hated her and never talked to her by the end of part one, who has tried to kill her multiple times, and who she has recently acknowledged as not a great guy, as someone she is ashamed to have feelings for. This is a girl with some serious self confidence issues when it comes to romance, and that shows in her feelings with Naruto as well.
Again, look at her reaction in 458 and 459 - she thinks Naruto is an idiot for loving her (indicating she doesn't feel like she's someone who deserves his love), and that she's caused him the most pain. This is reinforcing her words in 297, where she clearly feels that she doesn't do enough for Naruto. At the summit, she even asks Naruto after his reaction - "If you don't like me, then just say so!", indicating that she probably doesn't even believe he's still in love with her - not because of his actions towards her necessarily, but because of her own lack of self worth in this particular regard.
Man...this post i just full of so much win!