Has she a provisional driving licence? Could have bought her The Highway code to study for theory test. No joined up thinking. I think the car idea was just to satisfy the questions from ifam or 'trolls'!
I wouldn't put it past Chris and Sarah to have pre warned Isabelle that she isn't really getting a car but they had to pretend they are getting her something substantial. I think she knows pretty much everything including the state of their hemorrhaging finances. It would explain her reactions.
She seemed genuinely excited by the presents she knows she gets to keep and more matter of fact about the over rehearsed car key shenanigans.
Added to that her STEP dad giving it large about how she can't wear the clothes he would clearly have had a say in buying. It came across as a reverse psychology attack on the Ifam, a kind of look at how Puritan I am.
It all ties in together if you ask me.
They are right, we do only see 20 minutes of their day but we can read a lot into that 20 minutes