When Josh gets into the astral world, the first kid he saw is the lil Josh right? The one who pointed to him where to go. So, if lil Josh is in the astral world, is the "Adult Josh" is actually, some other guy who replace real Josh when he was little? And the other guy slowly forgot that he's not the real Josh, hence why he didn't remember Elise at all.

Maybe the old woman wanted to destroy the image because she wanted to forget about astral travelling and live a normal. If she remember, there's a chance she may accidently travel later, indirectly letting "others" to grab Josh's body.

And who is the big guy? Why did he walks around the house? Is he the guardian of the red door?

Did you notice, that Josh was actually facing the window in the kitchen, so chances are that Josh is actually yelling at his own reflection. So, he may already been possessed by the ghost lady, just that he didnt notice it