Heretics of Dune : Some Queries

[DMFW]

I'm in the process of rereading the original Dune series for the first time in many years and have just completed Heretics of Dune only to realise that there are several issues in the book I don't understand. Perhaps people on this forum can enlighten me?

*** Warning : There are spoilers here for people who haven't read as far as Heretics ***

My first problem is with Taraza's plan for the destruction of Rakis. As she is killed she thinks "I've succeeded" and we are then told that to remove the precient hold of Leto's dream time in the worms it was always Taraza's aim to have Rakis destroyed. But I don't see what the Bene Gesserit actually did to achieve this. It seems to me that the Honored Matres destroyed Rakis for reasons that had nothing to do with the Bene Gesserit i.e.

(a) They were afraid of Teg and his new powers which the Bene Gesseris had nothing to do with and didn't understand.

and

(b) Removing the source of Spice for the Bene Gesserit which would obviously have been a political goal but had nothing to do with Taraza's plans.

So I don't understand Taraza's plan at all. This brings me on to my second specific problem. What was the purpose the Bene Gesseris had for the Duncan Idaho ghola? I can understand how as a supreme sexual imprinter he would become a weaspon against the Honored Matres but how did the sisterhood know that the Bene Tleilax had built this ability into him? There are hints in the book that they didn't know what the Bene Tleilax had done but then on the other hand the way they were planning to use Lucilla suggests they did. I'm not entirely clear about this!

My final thought is about Lucilla's adventures on Gammu. As the escapees flee the compromised Keep we learn that Gammu society has already been heavily penetrated by returnees from the scattering. Lucilla seems to find this a surprise and yet every passing casual character encountered in the city knows all about it. It doesn't seem credible to me that the Bene Gesserit who are supposed to be supreme political manipulators could have been so amazingly ignorant of the true state of affairs on Gammu.

Perhaps all will be clear when I move on to Chapterhouse which I have just started but I don't remember it being clear when I first read it!

Anyone got any comments on this?