Thanks for your message, and sorry to hear of the issues you are having.
There are a few issue here you need to consider.
Firstly, it is liability. It seems you do not accept the charges for the speeding offence due to your car being faulty. If this is the case, you will need evidence of this, and need to prove it was faulty. If you are able to do this, then you have a viable defence to reject the speeding fines. You will need to take the car to a garage to obtain an engineers report which confirms the speedometer is faulty.
Secondly, if your letters were not received on time, due to no fault of your own you can make an application to the court for a statutory declaration. This effectively "resets" the fine back to the cheaper fine. But this will only be relevant if you are accepting liability but only wish to pay the cheaper price.
Lastly, regarding the amount of points, unfortunately if you receive 12 points then you will be banned under the totting up procedure. But you may be able to avoid a ban by attending court and pleading "exceptional hardship" I would suggest you instruct a solicitor to assist you with this.