Your question is a bit vague but the opera scene is part of the story, not a flashback. It's part of romancing the mother.
As for getting all the girls at the end, walkthroughs are available as free PDFs if you Google for them. Either that or avoid breaking up with anyone and use step back key (mouse wheel works in most cases) whenever you make a choice that loses you affection with the woman you're speaking with.
Although never true in life, Ren'Py allows you to take back what you say. It's the scripting language of forgiveness 😉