Sorry to hear the bad news about the car. It's a tricky situation. I had an issue with my roof leaking and like virtually everything the warranty wouldn't cover it. To be honest the nissan warranty is next to useless. I was lucky the garage agreed to sort the car out at their own expense and as a gesture of goodwill.
As I was not prepared for this I had done some research and gone to citizens advice who were great. I found that if the car is not fit for purpose then you have 6 months from the date of purchase when you can return to keys to the trade seller and get your moeny back. it must be a trade and not a private seller. in the case of motor vehicles not fit for purpose has to mean it's pretty serious and the car is terminal. I doubt very much whether your issues would satisfy the criteria unfortunately.
Sounds like it has now been quite a while since the date of purchase which will make things even more tricky. Sorry it's not good news.
I would still chase the garage, mention you are a member of this forum and you pass feedback back here. Also try the fit for purpose route and see if they fall for it, doubt it though and hopefully they may fix the problems.
I was luck yhope you are too