The stopover is not only for the day of arrival and I can vouch for that.
Sorry but this is just misinformation.
I'd be willing to bet a lot of money that there is no term to prevent one from booking a subsequent night as part of the free stopover.
The issue is simply Etihad trying to take advantage of joint bookings.
Its also apparent that Etihad now have a box to fill in who you'd like to share the car with on your chauffeur driver when you are booked on the same booking reference.
I used to share cars, but now I will be taking a car each just because of them asking this question.
It would be interesting to see Etihad's statistics for number of chauffeur bookings increasing after this change, but I as the customer don't expect to be nickeled and dimed.
If Etihad start offering a joint booking discount I'll tolerate this, until then I suggest nobody else does either.
If I'd processed 4 separate tickets under four different transactions and got 4 different booking references, I don't think I'd be facing this.