Wow, what a situation. I can’t fathom how a 5 seater works with a family of 7 but if you have made it work, that’s still an option.
I would definitely go back to the Motability Foundation and see if you can get dispensation to go for the more expensive car that actually suits your needs. Alternatively have a look at the Citroen Berlingo, it’s not got the greatest car looks but it does come with 7 seats and Citroen’s typical comfort. I would strongly suggest that test driving that and the Peugeot equivalent would feed your argument if you feel the 5008 is your only option and you’re willing to cover the rest of the payment.
When approaching Motability to try and get them to do something beyond their normal practice I’ve only heard success when you can specify a genuine reason why the cheaper version fails to deliver what you need. If you can justify rejecting through medical or practical shortfalls then your chances improve, for example the Citroen can’t provide the required range all year round. Even Motability’s own calculator states you can get 101km out of the Berlingo in winter, which would be enough to convince them taking kids to school requiring a recharge is an unacceptable requirement.
Its going to be a tough ask but if you can justify going electric because of fuel costs, range because otherwise the journey isn’t possible and some comfort/practacal/medical reasoning might get a favourable outcome. The problem is that without considered justification, it just sounds like ‘I want a nicer car for free’ instead of the cars you left me with a choice of were completely impractical.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark