Stick to asking them questions that you know they don't want to answer, rather than telling them things that you know that they don't want to hear.
Don't let them off with crap answers and make them be honest by standards that they profess to keep.
It is a fine line to tread., especially when both parents are full on JW nutters. Sending them an email crossed that line.
You need to get your parents off guard. They have to know that they need to defend their descision to remain with a church with a history of prophetic failure.... Failures that they know about, but pretend/prefer to ignore. Try to bring up statements that you heard them make.
I didn't say it was easy.
Take it sloooooowly. Your parents are psychologically damaged from many years in the cult, so don't expect that you can extract them in a couple of weeks.