Q: In IW, Thanos used the time stone to reverse the time so he could the already dead Vision, and it didn't cause any time parallax. Why did no one use time stone to save Ironman's life in EG?
A: It's because even if you save Ironman, it will still not change the fact that Thanos will eventually win the war. Among the 14million possibilities that Doctor Strange has seen, Ironman's sacrifice is a must for that one win scenario.
Q: How did Thanos bring his army to the future?
A: There is a guy called Maw in his army, he was a great wizard. Thanos himself was a brilliant genius as well. Those two easily reverse engineered and mass produced Pym Particles.
Q: What about those people who got dusted? What did those five years mean to them? Why didn't they grow older when undusted?
A: Yes, those people whom was lucky to survive the snap are 5 years older than the people who just got back. The reason Spider Man saw his friend again in high school at the end was simply because his friends was unfortunately also dusted like Spider Man was. Of course, there are people in his grade whom didn't die and they are probably already in colleges by now. To those dusted people, they had no conscious in these past 5 years. They didn't know what happened. It's as if they had just woke up from a long sleep. The only one who was aware about how many years has passed was Doctor Strange, because he has already seen that when he was time mediating on Titan. Parker's reunion with Nat was a touching moment. There are also people whom indeed moved on but suddenly was reunited with their lost ones. Yeah it's kind a complicated world now.
Q: What if the mouse didn't press the button to turn on the quantum machine, wouldn't that stop EG from happening? Isn't this a bit too much a coincidence?
A: Yes, the MOUSE SAVED UNIVERSE. Among the many realities in those 14millions possible futures Doctor Strange foresee, the mouse failed to press button and thus the heroes failed in those futures.
Q: EG's plot, is it a parallel universe or a closed time loop?
A: Nope, not a time loop. Both Ancinet One and Hulk were right. You can't change the future by simply going back to past. But it's possible to create a different alternate future. It's not butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam.
Q: There were some metal smashing sound when the movie ended. Was that easter egg? or just a tribute to Ironman, or maybe an implication that Ironman will return?
A: It was our way to say goodbye to him.
Q: Why there was no Ironman's body in his funeral, only his arc reactor? And is there any secret messages for bring back that kid from Ironman3?
A: We just feel that he should participate in Ironman's funeral. As for whether he will appear again in future, who knows.
Q: Why didn't Black Widow get a funeral as well?
A: Did you forget when the heroes where mourning for her after when they returned from past? Maybe her funeral happened off screen. Maybe it will be shown in future installment, because there are still tons of stories in MCU that are waiting to be tell.