Look at my grammar book correct sentence:
1.Direct = She wrote, “Negative emotions are dangerous and they cause illness.”
Now I have changed the sentence but I am not sure whether it's correct or not.
Look at the sentence which I changed the above one:
2.Direct = She had written, “Negative emotions were dangerous and they caused illness.”
Is my self made (#2) correct as well or not?
If you want to change the tenses, make it indirect.
Yes - we back shift the tense for reported speech, not for the written word.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.