I think the first thing that would likely help would be to extend his first A time to closer to 3-3.5hrs I suspect he is Undertired for that first nap and then gets overtired from then on because he hasn't had a restorative nap but still does a full A time after the short nap. You may need to extend it by just 15 mins every 3 days or so to avoid him getting OT. Right now he is having a long period awake right before bed after a short nap and that means he goes down to sleep OT and therefore sleeps fitfully and wakes easily. As he isn't able to self soothe (the car seat is still a prop as he has the motion and sound to lull him to sleep) he then requires your breast to get back to sleep.
Have you read about the Gentle Removal Plan for weaning a feeding to sleep prop? Here is the link to the info.
Gentle Removal PlanI think this would be a good plan for you as your LO has never been an independent sleeper.
Another alternative would be to use PUPD which may be quicker but would involve much more crying I expect.
Personally I would tackle the naps and daytime routine issues first and then when he is better at going to sleep without your help in the day you may find the night wakings reduce on their own. It should be a quicker process to then wean the night feeds as he will have the skills to put himself back to sleep.
Eventually you are aiming for a routine with two long naps of 1.5hrs or more and 3 A times of around 3-3.5hrs.
What do you think?