"The brothers in Bethel/Headquarters seem very sincere and loving - what possible motive would they have in trying to deceive people, what do they get out of it?"
Personally my answer would be power, as they do not seem to exactly raking it in but how would you answer?
Power and control, IMO. While I'm sure there are many at Bethel who believe that this is "the Truth" and being guided by Jehovah, I don't believe that the Governing Body can still be this clued out. They are more than aware that their policies and doctrines are put in place by a 2/3rds majority vote, and not because the Holy Spirit was guiding them. They were made aware more than 30 years ago that Jerusalem was not destroyed in 607 BCE, but in 586 BCE, yet they've purposely kept this false doctrine in place and DF'd anyone who doesn't believe it, for the sole reason that it would upset the apple cart and their position as the 'F&DS'.
It's one thing to teach something in error. It's quite something else to continue to teach false doctrines when it's been proven that they're built on sand and should have been dropped decades ago. While they may comfort themselves with the idea that the end justifies the means, how can they kid themselves by thinking that Jehovah approves of such underhanded tactics? In reality, they've become the 21st century version of the Pharisees, apparantely unable to see the delicious irony in that.