At one point Kelly and Max are wondering what they will do about money. Later on, Max flings a bundle of notes at her which he either had at the beginning or was able to obtain later.
At one point Max suggests setting fire to the car as it will mean Kelly and he will be presumed dead. Someone capable of woodcraft and driving a car would realise that a burnt-out car with no human remains would suggest nothing of the kind, so this is hardly credible.
When Max's Mum realises water is coming through the ceiling, she doesn't attend to the situation. Rather she calls out repeatedly despite the possibility that Max might be dead or the ceiling could dome down. Also, the bath isn't full, so how come the leak?