Answer: She wanted to be with her mom who had passed sway.
It's never stated that her mom had died. There could be a possibility that Charles and her mom got divorced. The first two answers are more likely.
I've heard the interpretation that Delia really is Lydia's biological mother, but they are very different and don't get along, so Lydia pretends that Delia is a stepmother.
Question: When Adam and Barbara start to age and die near the end of the movie, I don't get how they didn't "die" again. How did they end up completely fine at the end?
Answer: Otho is attempting to exorcise them (remember how the afterlife janitor said exorcism was death for the dead), but Beetlejuice interrupted the ceremony, restoring them.
Answer: It's not completely clear what was happening to them but it's somewhere along the lines of the ritual making them corporeal or even partially resurrecting them but the ritual wasn't done properly so their bodies started to decay. Beetlejuice reversed that process and turned them back into ghosts.
Question: If the Maitlands aren't dead at the end and the Deetzes agreed to share the house, then how did Lydia fly while dancing?
Answer: The Maitlands are still dead. They are still ghosts.
Answer: The Maitlands are definitely still dead. Absolutely nothing in the movie indicates they came back to life. Lydia flying and all the things happening around the house at the end are the Maitland's doing - they're having fun with Lydia using their otherworldly abilities because Lydia's doing good in school.
Question: At first, the football team doesn't realise that they died. So why do they think that Juno is their coach?
Answer: They are very confused and probably not the sharpest tools in the shed. Juno probably told them they should do what she says and they immediately associated that with a coach.
Answer: Right before the Maitlands walk in, one of the football players asks, "coach, where's the men's room?" After which Juno finally loses her cool and dismisses the team out the door. After Juno's conversation with Adam and Barbara, one player comes back in and says, "Coach I don't think we survived the crash" suggesting they realised they were dead.
Question: Is there any reason besides plot convenience that the Deetzes didn't enter the attic during the three months that the Maitlands were away from the house to meet with Juno? I realise they don't have the key, but seeing as how they were renovating the entire house anyway, it seems like they would have had no problem just knocking the door down.
Answer: No reason was given, but they probably felt no immediate need to enter the attic. I've never been in my own house's attic. As you pointed out, it's really a matter of plot convenience.
Answer: After the dinner scene when Otho asked where they hid and Lydia said "the attic" Charles replied that the attic was locked. So it seems like she never told them she had the skeleton key.
I addressed this in the question. A key is not required to get into the attic because they could just break the door down.
Except they didn't break the door down. Delia kept banging on the door until it opened. If they had broken the door down, there would have been some damage.
Answer: This is purely for convenience. It's always bothered me. Like since the early 90s when I first had a VHS copy to rewind. That whole house has been gutted and rebuilt but no-one got in to the attic for 3 months? That's BS. For one thing not only would someone like Charles Deets want to see every square inch of his property, but a major company/contractor doing a remodel of that size would have at some time needed access to and been on every square foot of that house.
Quite often, people don't think about the attic along with the rest of the house. Many of them won't be going into the attic every day, not every month, maybe not more than once a year.
Answer: She wanted to be with the couple, she did not think of the repercussions.
Phixius ★
She was incredibly depressed. She wasn't happy anywhere and she didn't fit in with her own family or anything they were trying to do. She was a goth girl. The realm of the dead and the macabre seemed more a place for her. When one sees the "Netherworld," it kind of seems like a more interesting place. But... upon closer inspection, it's not really all that different from our world.