Welcome to the forum
Thank you, glad to be here. This place seems like a goldmine of good Dune discussion, seems much more active and in depth than the other forums.
- and that "somehow" is VERY easy to explain - the oracle sees the future and then does something to bring that future to be. FH wasn't talking about some magical force the oracle exerts on the universe. If the oracle no longer has any way to exert force upon the universe (as the worms cannot) then this grip ends. Humanity was set free from Leto's visions as soon as it survived the Famine times and undertook the Scattering.
I personally want prescience in the Duniverse to be as least 'magical' as possible, but I think there's still a lot of thinking to do before it can be explained - it cannot be summed up with simple common sense IMO. First , just look at the quote at the start of HoD chpt 47:
We are not looking at a new state of matter but at a newly recognized
relationship between consciousness and matter, which provides a more penetrating
insight into the workings of prescience. The oracle shapes a projected inner
universe to produce new external probabilities out of forces that are not
understood. There is no need to understand these forces before using them to
shape the physical universe. Ancient metal workers had no need to understand
the molecular and submolecular complexities of their steel, bronze, copper,
gold, and tin. They invented mystical powers to describe the unknown while they
continued to operate their forges and wield their hammers.
-Mother Superior Taraza, Argument in Council
To me this doesn't leave much room for interpretation, except that Taraza might be mistaken. I also recall from the second novel, the preface by an historian saying 'completely accurate and total prediction is fatal,' referring to the downfall of Muad'dib, who couldn't escape his doom. I think what we're looking at here is an extrapolation of the observer effect (or maybe even the Heizenberg uncertainty principle, or the Schroedinger's cat thing): merely looking at the future will affect it. Obviously the oracles can see different possibilities, so they can't 'all come true,' but I think the point is the more closely you look at one particular path, the more locked-in you become. This the trap Paul fell into, but Leto II avoided before finally using to his advantage.
I suspect prescience works something like: you have minds like Paul or Leto II, enriched by the knowledge of Other Memories (male and female), mentat training, and spice. They perceive a 'higher dimension' (the books use those terms), as others can (eg. Guildsmen), they can sense the 'currents of Time,' but unlike the others they are able to make sense of it all because of their deep understanding of human motivations/trends (and in Paul's case mentat calculation). However once they have a clear picture of what forces are at play, somehow through their vision
alone they can affect the 'external probabilities' ie. lock the future into one path, or at least make certain things more likely. Then of course they can take appropriate political actions to further solidify things.
Also, the Golden Path is set in stone, cannot be undone. The whole point of the Golden Path is that humanity can never again be threatened by one single danger (hint: the Scattering IS the Golden Path), and it's future could never be "locked in" by a prescient ever again. I see where you were coming from with this idea, but it relies on 2 things that simply don't pan out (the way that precience "creates" the future, and the nature of the GP which is the opposite of what you were thinking).
But the Scattering was not achieved automatically. At the moment of Leto's death, everything was set in motion, but it would still take a long time to go beyond the point of no return. Therefore Leto II needed to maintain some hold on destiny until his aim truly was achieved and no longer 'undoable.' I suppose there are some other interpretations of his 'unending dream' and its function, like perhaps to create a nexus blocking other prescients, or to influence the behavior of the worms. But the weight of evidence from the text and narrative choices just leads me to conclude that
1) prescience alone affects things in ways not understood (i mean if it was very simple the BG wouldn't be confused about it)
2) the pearls of consciousness and their 'unending dream' were a vital part of The Golden Path as they continued to influence history (not so much 'pulling the strings' like the God-Emperor, but just 'holding the strings' in place, steadily deflecting the currents of time).
Otherwise the last chapter of Heretics is just a big red herring... not the best way to end a novel!
Good to have some debate though, different veiwpoints are always nice. I'd recommend checking out some of the threads in the Golden Path subforum - we've been talking about exactly what you're saying here for years and have come to our conclusions based on massive piles of evidence, it's a good read.
Yes I've read a lot of them but there's a lot to get through! A good resource would be a compilation of all the excerpts on prescience/oracles found in the novels, wonder if anyone's made one. Sometimes epigraphs at the start of chapters contain massive clues/indications that flew over one's head on first reading.
held humanity in some kind of hold, whether it was true or not. Someone here (I forget who at the moment. Help, someone?) pointed out that the destruction of Rakis & its worms did in a sense free the BG psychologically by removing the source of that belief. 
