Good post, rhpt, and welcome!
Your summary sounds reasonable, as best as I can remember since reading it last. I'll try my best with some of your questions:
[*]At the end of the Heretics Odrade says the Bene Gesserit have to re-think its history due to something Miles discovered. What did Miles discover?
Is there a specific quote you're referencing here? Without knowing that, the best I can say is that Miles had a theory about how to be defensible, which involves always being mobile and never being holed in. I assume his understanding of this was groundbreaking, and probably in some way relates to other gifts of his. If this is what you (and Odrade) are referring to then the BG need to rethink their pattern of situating themselves in one region of space and worrying about one Empire and the lines of power that run through it. All of it circles around the spice, which in turn sits on one planet. In short all of this was locked in, and was just begging to be whacked by some bigger fish. Mobility requires completely rethinking what for the BG had always been a clear way to control things. If there is such a thing as control, it could no longer be done in this way.
[*]Wazs Taraza's plot part of the Golden Path, or was her plan to break humanity from the Golden Path?
Leto seems to have refused to peer into the far future except *only* to verify that his path led to humanity surviving. Beyond that he at least claims not to want to know the details, perhaps in part for fear of locking humanity into his vision. To the extent that Taraza's plan was a furthering of removing all the eggs from one basket for the BG
, I doubt that Leto II would have specifically created a future for them of his choosing. His plan was to safeguard all of humanity, but not specifically to save the BG as far as we know.
[*]Was Leto aware that the GP would produce the Honored Matres and his Fish Speakers/Siaynoq would be perversely twisted?
I do think he had definite notions of how Siaqnoq would be used in the future, but I personally think he came to this determination through logic and mentat reasoning, rather than prescience. I don't think he used the powers of the oracle to look thousands of years into the future to look for cultural details. Whether he might have surmised through logic that the HM's would become what they did...well I guess it's not that much a surprise that once 'sexual technology' (so to speak) exists it will eventually get out of BG hands and be used in all manner of ways. Secrets can't remain secret forever; if knowledge exists it will get out and be used by many. So yeah, I guess he'd have figured someone would misuse sexual slavery, but I also don't really think he specifically foresaw them specifically.
[*]Was the destruction and death of the worms part of the GP?
The GP was only about prevent humanity's extinction. IMO it also involved certain 'lessons' to lead us forward, some of those lessons genetic, but the GP did not seem to detail any specific measures that would be taken in future. Although it's true that humanity would sort of be screwed if the worms died, the GP doesn't seem to strictly speaking be about the spice always existing. I assume that, technologically speaking, a substitute would eventually be expected one way or the other...
Now regarding whether Leto II's vision specifically did involve this future and Taraza's plan, it might have, but that doesn't mean he was aware of it. He did choose a future, but chose not to examine it too closely other than knowing it worked.
[*]Did Leto "push" the Ixians to develop no-technology with his request for devices to write and shield his diary?
I think he did. I think he also pushed the INM so as to kick off the Scattering. Its invention is probably why he timed his death when he did.
[*]Was the Uber-Idaho part of Leto's plan? Is that one of the reasons he kept producting Ghola Idahos?[/list]
Well I think that was a BT plan, but are you asking whether he was aware of it or even supporting it? I don't really know the answer to that. We run the risk sometimes of assuming Leto II was just omniscient, which he wasn't, and he did like surprises. He did seem to definitely have a plan for Idaho that involved (a) having Idaho keep him in check somewhat, and especially to gauge how much he had become The Worm. And his plan for Duncan also involved Siona and genetics, of course. I'm not sure what use he would have had to hope to produce a special uber-ghola.