3 out of my 4 cats are virtually indoor cats (age rather than me not letting them outside, they're 19, 14 and 10), the 10 year old cat (still refered to as the kitten, lol)never went out of the garden anyway, but out of 15 cats (those that I've owned myself and those that I grew up with only 3 have been hit by cars (all 3 were instantly killed)- 1 of my cats had been hit by a car before I rescued him and hadn't seen a vet as a result his pelvis isn't aligned properly but other than that he's fine, he does wheeze a little due to a broken rib that never set properly and he snores loudly (but that could just be how he always was). He has almost full use of his back legs, but even after 6 years of me owning him he still prefers to use his front paws to pull himself up the tree in my garden.
I hope your cat pulls through, it's awful when somethings happened to them and you can't make it better straight away. Fingers crossed for you and the cat.