Happy to hear you had a great time.
As for Osborne Lights, the dessert party will need to be over by fireworks time but the lights themselves should be unaffected, they will close Pixar Lane for about an hour and a half, so if fireworks are at 11 pm, Pixar lane will probably close between 10:15pm until 11:30 pm
Keep in mind that the park hours got extended but not the Osborne Lights hours. So for example, when the park closes at 11pm the lights still say closing time of 10pm, and I still think they will stay on the extra hour, so they will turn off at 11pm
How will that work with evening EMH I don't know, I'll try and go to DHS this week and I'll ask