Question: How did English Spider-Man (the rebel one with the guitar) get back to his own dimension after leaving his watch thing for Gwen in her dimension?
Question: How old is Mayday supposed to be in this movie? It's supposed to take place 16 months (one year and four months) after Spider-Man: Into the Spider-Verse, which makes no sense. Peter would've had to re-marry MJ in his universe just like that and nine months later, their daughter is born, so Mayday is three months old?
Answer: Who's saying they got married first? They reconciled, she got pregnant at some point, and they got married whenever. Do we even know if they are married? Mayday could be as old as 7 months. She's crawling and seems to have basic motor functions like pulling her hat down, so she's likely at the older end of that scale. While she's big-ish, she's not talking or anything, so she can't be that old. Plenty of younger babies still have a lot of hair, plus we can't be 100% sure how his universe works, in terms of child development or indeed be sure that time passes at exactly the same rate as Miles'.
Okay, you made a fair point on the whole marriage thing.
Answer: She is an infant; she appears to be around 3 or 4 months old.
Answer: Hobie or Spider-Punk gave Gwen's father a watch he'd built himself. It's fairly safe to assume he built more than one.