Hi Tasha, there is a phone app you can use (free) called Clinometer to see if the seat is at a suitable recline if that might be an issue. Extra inserts aren't recommended if they aren't designed and tested with the seat by the manufacturer. If you do manage to return the seat, the SnS Compaqs are good but the Infasecure Neon and Kompressor models are great too, they are designed for smaller cars so should fit nicely and are much more reasonably priced than SnS seats. The Neon and Kompressors are the same seats but the Kompressors have narrower bases and extra padding for newborns. Hiring a capsule is a good idea but you could also hire a seat that will allow you to rear-face much longer as that is the absolute safest option. The law states it is legal to forward-face from 6 months but that is not the safest way to travel for babies as their bodies aren't strong enough to withstand the forces in an accident yet. Being 'big' or able to hold their heads up etc are not good indicators they are safe to forward-face. The Infa 0-4year seats will now allow to RF until 30 months for average sized kids you won't get that long from a capsule.

Also check the manual as to whether or not it can touch the front seat or not. some seats require a gap and some are allowed to touch as long as it doesn't alter the angle of the install. It is never safe for the seat to touch the front seats of the car if the car has some types of advanced airbags, you need to check the cars manual to find that info.

Sometimes putting the seat in the middle and lifting the headrests on the front seats will make the seat sort of 'puzzle' nicely between the two and still let the seats be back enough. Good luck I hope you get it sorted soon.