You're probably right. I'm not exact in my memory with DS9 original airdates and INS premiere and Dominion War timeline.
Even still, post-war or not, O'Brien would be an easy choice for an appearance at the wedding. Of all crossover characters, being invited to a wedding for old friends is a
perfect setup. No forced explanations or excuses.
Of course, I was also surprised that none of Worf's Enterprise friends came to his wedding to Jadzia. But hey, what can you do?
The deleted Quark scene in INS wasn't at the wedding party. It was at the end right before beam-out, Quark showed up with some Dabo girls in tow. Really odd appearance out of nowhere.
I don't have any links, I thought it was common knowledge regarding deleted scenes from INS. I think wikipedia or Trekcore or Memory Alpha has some info, it's not that obscure.
You seem to keep missing a point here: the Riker/Troi wedding was in Nemesis, not Insurrection. And the Dominion War was still going on during Insurrection, but was over before Nemesis.