It sounds like what you are currently doing is working out fine. I went all out against the WTS and destroyed my family ties in the process. Other friends I grew up with keep their thoughts to themselves and it works a lot better. What you may want to do is let her know over time that you are no longer active, better she finds out from you than from others. Just make some excuse about work load or something, rather than admit you do not believe it.
You mother has suffered a lot, and the hope of paradise is possibly what gives it all meaning. It may be best to leave that hope with her due to her age.