I wonder if they wrote to you because they are struggling with the situation.
They are obviously angry and you are the logicial target.
I would write back and say "Even if I'm 100% provably correct, you will refuse to believe me because you have been trained to only believe the WT, even if they make no sense and can proven to be incorrect", then list the reasons why you have left.
For me it was the flip-flops. There is nothing solid to base your faith on because it can change at any time and you have to accept it.
100 year old teachings, preached as 'truth' from door to door, dismissed in a single paragraph. The WT today is not what I got baptised into - it's teachings are completely different.
But don't be angry back.
Don't list 1000 things, just some that you know will have an impact. You can say "there are a hundred more reasons, which if presented in a court of law would be conclusive evidence of their lieing", or something.
You know how best to respond - be smart about it, you might not get another chance.
Splash