A Guest in the House question

This book was so interesting, and I want to talk about the ending NOW instead of waiting for deconstructions in a little less than a month.

This thread WILL have spoilers, so be warned about reading it before reading the book.

(view spoiler)[So to me, there is one of four main possibilities:
1. The woman in the lake is not Sheila, but was a wife of David's from before Sheila (This to me explains why one of the dentists said that David's wife killed herself, but also David said it was cancer. There's the chance he's lying, but also there's a chance that he's simply remembering another wife. There's a point when Abby is going to talk to Crystal's teacher that he accidentally said Abby's teacher instead: He gets people/names mixed up). This would also explain why another woman who apparently was near by and hiding was actually Sheila.

2. The woman in the lake is not Sheila, but truly believes she is. This could mean she is the Lady in Grey, Abby's imaginary friend from childhood, or she is simply a ghost. This one I like the least because it doesn't explain why Crystal was drawing HER in the lake, and said she saw her. Also it doesn't explain why the neighbor looks NOTHING like the Sheila in the photo Crystal kept under her pillow. This would explain why she is so possessive of David after she learns that he may have thrown out all of her old paintings, and it would explain that the neighbor IS Sheila and just... faked her death? Didn't die at all, just did a quick divorce that Crystal didn't fully understand? Had an agreement with David to leave and never come back? None of these really make sense and this is my least favorite option.

3. The lady in the lake is Sheila. The neighbor is lying and became concerned when she saw the dead body (in theory she was the one with the counterfeit money, and she was just trying to do a quick con against Abby? Kidnap the dentists' kid? Get a ransom? A little bit of blackmail as a treat?). This makes more sense to me and I think I like the best. It explains why Crystal drew the lady in the lake instead of the neighbor, the picture looks more like the ghost than the neighbor, and locals said the house was empty. Well, if it was meant to be empty, that's a perfect hideout for a con artist.)

4. The lady in the lake is not Sheila, and simply wanted to trick Abby into giving her more bodies for the lake. The neighbor is Sheila. Crystal was being lured to the lake by the ghost as much as Abby was, just in a different way. (hide spoiler)]

I think the hardest thing to parse in all of this is who is lying, who is telling the truth, and when, because David gaslights Abby a couple different times in the story, and we learn we can't always trust outside sources either so... yeah, I want to see all the theories, the breakdowns, and the details!