Their greatest strength is their ability to predict the flow of fate and causality almost to the tee...
We don't know the extent to which individual members of the God Hand are able to "predict" things. The principle of causality guides a lot of what goes on in Berserk's world, and it is wielded by the Idea of Evil, the "God" that the God Hand serves. Whether it shares knowledge with its agents or they have this ability themselves (or only some of them have it) is unknown. You'll notice I haven't referred to "fate" here. That's because it's not a mystic force in Berserk. There's the notion of fate, as in the expression "meeting one's fate", but there's no Fate like in Greek mythology where things are predetermined and unstoppable. That role is played by the Idea of Evil's control of the world through causality.
I believe that they can fully predict events within the Mortal World (Corporeal Plain/Human World/Earth) and the actions of a human right down to the last molecule...
This is not what the story tells us. Rather, the Idea of Evil's control on the corporeal realm (and on mankind specifically) is so great that a normal human cannot escape its grasp. We are not led to believe that members of the God Hand know everything before it happens. Note that if they did, it'd make Sonia's role in the story completely pointless, since she works as Griffith's seer. That doesn't mean members of the God Hand can't feel things, like certain big events, or have other types of supernatural acumen, but likely not in the way you depict here.
However, the flaw with this ability is predicting the fates of individuals who are able to exist beyond the Mortal World i.e. the different layers of Astral Plain...
Examples of such individuals are as follows:
ē Branded individuals, namely Guts and Casca, who are said to live within the Interstice because of their brands. Skull Knight also states that he lives within the Interstice as well.
If you read Guts' conversations with the Skull Knight, you'll find that people who exist within the Interstice between the Corporeal and the Astral realm (or plane if you want, but not "plain") are not at all outside of the flow of causality. But they are able, during particularly momentous events (which are called "Junctions of Times") where a lot of threads come together, to interfere in a minor way. Note that being in the Interstice isn't enough in itself. Apostles also exist within the Interstice and they're very much under the Idea of Evil's control.
ē Magic-users such as Schierke, Daiba and Flora, who are able to release their Ethereal bodies into the Astral Plain at will.
Daiba is not able to do that as far as we know.
ē Other God Hand members (I think)... Although they work together, I donít believe they can accurately predict each otherís movements and actions based on what Slan said to Guts and SK in Qliphoth.
While that is likely true, keep in mind that Slan is not a reliable source of information here. She told the Skull Knight she didn't care about the others because 1) that is true 2) she definitely wasn't going to tell him about it anyway.
Not to say they canít predict Ehtereal beings, they can probably predict their fates 99% of the time...
That is in fact unlikely. The Idea of Evil's power (and that of its servants) is entirely derived from mankind. Whether it is able to influence beings such as elves or spirits like the Lady of the Depths is not sure at all.
An example of this flaw is highlighted during the Eclipse where they failed to foresee Skull Knight gate-crashing the ceremony and rescuing Guts and Casca...
That is because the Eclipse was a "junction of times", a moment so significant that unexpected things can happen. Same thing for the Occultation ceremony or the advent of Fantasia.
I also believe that being a magic-user gave Ganishka the confidence to oppose the God Hand even though he was an apostle: a servant of the God Hand...
Ganishka was playing the role he was meant to. He was on rails all along, as the story showed. His life and death served the God Hand's needs to perfection.
Skull Knight created the Sword of Actuation to kill the God Hand (specifically Void)
No, not "specifically Void".
because he had to use it earlier than expected in Qliphoth, Griffith was able to foresee and counter SKís attack atop Ganishka...
From his dialogue, Femto didn't foresee it so much as logically guess it. Again, that event was a Junction of Times, a moment where unforeseen things can happen. And because the Skull Knight doesn't waste his efforts on futile endeavors, it was logical that he'd try to strike at that time. That's what Femto tells him. Also worth noting is that because the sword uses beherits, which are irremediably tied to the Idea of Evil, the Skull Knight's attempt might have been doomed to fail to begin with.
However, if someone were to create another weapon to kill them, the God Handís flaw may render them unable to take proper measures to counter the weapon, thus defeating them...
You're making it sound real easy to create a weapon that could kill the God Hand. Also, do you really believe that if a weapon were to strike down Femto, it could be anything other than the Dragon Slayer? Because... no. If it has to happen, it'll be the Dragon Slayer.
Which is why I believe that this may compel the God Hand to attack Elfhelm, which is home to a number of Ethereal beings and magic-users.
That's somewhat of a leap in logic. If they were to attack for that reason, they could have done so already. Also, no need for the weapon idea to justify their hypothetical decision to get rid of dangerous magic users.
When Femto was incarnated he got a new body of flesh aka a physical one. Now pretty much nobody can hurt him for he is the absolute one.
Nobody can hurt him because he's a member of the God Hand.
What if someone (with a huge sword that cannot be called a sword) remembers that name that was mentioned once in the eclipse was to say it aloud? Would it work?
Probably not. Otherwise the Skull Knight would have tried.
Now what makes me put this theory in the funny/probably-not-right section is that the god hand are not (at least I don't think so) pure astral beings. As in they were human before vs the other creatures that have always been what they are.
Other than Femto, the others don't have corporeal bodies. They exist solely within the Astral realm. But what you mean is that whatever rules apply to certain astral beings, like Merrow, might not apply to human-created magical entities. Which is indeed very possible.
P.S. By the way, Griffith's main flaw is that he shares his body with Guts and Casca's son. And that the boy would probably not let him kill them.