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i don't really care about Vernon... and i don't think he would have gone
You bring up a point, which may explain your mixed feeling towards Petunia. Vernon never wanted Harry in his home and went out of his way to make Harry as uncomfortable there as he could. Vernon did not care what happened to Harry as long as he was finally rid of him.
Petunia could have done much much better by Harry. However, she was also in the difficult situation of keeping Harry in their home despite the fact that Vernon did not want him there.
Some, though not all, of Petunia's mistreatment of Harry was an attempt to make Harry more presentable to Vernon. Petunia's haircuts reflected her conflicting goals. She kept Harry's bangs long to cover the scar - because she desired both to keep Harry safe and to keep her family safe when they were with Harry. She tried cutting the rest of Harry's hair short so that Vernon would have one less thing to dislike about the boy. Harry's punishment in COS after pretending to perform magic was because Vernon had threatened to kick Harry out if he performed (or possibly even threatened to perform) magic.
That said, Petunia may have been able to give Harry treats, but didn't. I say "may have been able to" since Petunia would have had to have done so in a way in which it would not have gotten back to Vernon. Harry, despite Vernon's efforts to make him downtrodden was very outspoken. Dudley was half bully and half normal brother-like creature - both halves unlikely to keep Harry getting a treat secret.
We learn what "Remember My Last" meant - the letter that DD had sent Petunia denying her entry into Hogwarts. It is hard to know how noble or how selfish Petunia's actions were in light of that. Petunia knew that giving Harry stay there would both keep him alive and bring danger on her family. Petunia also knew what Vernon's reaction would be if he found out about her desire to attend Hogwarts. Snape may have wanted both Lily and Mulciber (according to JKR), but Petunia wanted both to keep Harry safe and hold onto Vernon - also two conflicting goals.
What we don't know is the influence of their stay under the protection of Dedalus and Hestia.
Dudley had had bad experience with wizards before hand - with Hagrid's tail and the twin's candy - but it was him who decided to put his life in the hands of these wizards. Part of the reason was because he feared the Dementors more than the Wizards and part of the reason was because Harry saving him may have given him the impression that not all wizards were bad (as well as dementor memories reminding him of his own episodes of bullying). Dudley, of the three, would have been the most receptive to Hestia and Dedalus's attempts to make them comfortable. Dudley would have lost his fear of wizards and would have known at least two people at Harry's wedding besides the groom - candy boy, room blower up man, cat lady, and pig tail producing big guy. Dudley would have found out more about what the fight was about - because there wasn't much else to do and this would be more understandable than trying to comprehend what Quidditch was.
Vernon only went with Hestia and Dedalus because of Dudley - and probably would have continued the debate whether or not he would have been better off if he had not gone the whole year he was under protection. Vernon would do nothing but complain both about the state of his care and how, in his opinion, Harry had brought this danger and inconvenience upon them. Vernon would have emerged from the experience even more convinced that they should have never taken Harry in.
Petunia would have been filled with guilt and conflicting loyalty. Vernon would have made her feel like she had brought this down on the family. Petunia would also chastise herself for not wishing Harry luck like she had tried to but didn't. Petunia would become upset at the talk about how noble the cause was - probably admitting both her desire for Lily to stay with her in the muggle world and for her desire to go to Hogwarts to be with Lily in the process. Vernon would become very upset so they would wipe his memory - but Dudley would know.
Seems that a lot of the story happens off stage. The year in care with the wizards while Harry was trying to find Horcruxes would have impacted the Dursleys in ways we could only guess at. What happened there would affect both whether Dudley and Petunia attended the wedding and their understanding of the wizarding world when they got there.
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But i hope that when the Dursleys went into hiding with Dedalus Diggle and Hestia Jones, that Dedalus and Hestia told them everything Harry had been through ; all the voldemort encounters, explained about the OOTP, maybe spoke about sirius and showed them that he is a wizarding hero.
While Hestia and Dedalus was doing that, Dudley would not have much choice but to listen - there being nothing else to do. How much would any of this matter to Vernon? He would hate Harry even more for getting him involved in this.
Even if they told him of the Muggle killings the DE did for fun, Vernon would still see himself as immune (if Harry hadn't involved them in this). Vernon lived in a quiet "respectable" neighbourhood of home owners where any stranger walking down the block was immediately suspect.
The other question would be whether Vernon was still alive by the time the wedding occurred, because of his unhealthy lifestyle - and clogged arteries.
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I want Petunia to go because it's her only nephew and just for Lily's sake. ...
If she just attended the ceremony and maybe left at the reception, that would be acceptable. But i hope she maybe met someone at the wedding who was like " hi, your Lily's sister?" and started to her about Lily. I mean even Harry could do that... like maybe post DH write a letter to Petunia telling her about Snape's memory.
Petunia hated Snape because she blames him for taking her sister away from her. It was Snape who first told Lily that she was a witch and about the wizarding world. Petunia, I think, hated James for pretty much the same reason.
Petunia had felt unwelcome in the wizarding world since receiving that letter and wondering how welcome she would feel may have made her hesitant to come. I think people who knew Lily talking to Petunia about Lily would be a nice gesture. The vase that Petunia had given Lily for Christmas just before she died is proof that Petunia still cared for her sister. I am not even sure that Vernon knew that she had given her sister a Christmas present. Thank you for thinking of it.