so in the past, the world was indeed in a similar state to how it is right now, wow.
Yep! Very curious to hear about some ancient history now. Hope the old folks will be in the mood for tale-telling.

the hype for the elf king keeps increasing, saying he's hard to deal with makes it even more intriguing.
Indeed. Will he
just be whimsical, as all elves are, or will he also request
something of Guts in return for his help?
magic users are the ones who kept the world at bay and kept humans safe, now they're frowned upon by many, thank you holy see.
Well who do you think is behind the establishment of the Holy See, if not the God Hand? There's no coincidences here.
it explains why magic users are so rare; they apparently can only be found near these trees, because the trees influence their power and knowledge of magic.
No, no, magic users have become more rare in general. They're not naturally limited and obligated to stay near the trees, it's a choice some make. They become guardians and dedicate their research to those trees' power.
I'm not totally clear on 'The road of Elves' vs 'The Road of dragons' but what I gather is that the astral world naturally has some small overlaps from time to time, and this explains the reare sightings of elves and other astral creatures, especially in a nature areas like the woods (basically explaining superstition in this world).
Yes, this is essentially correct. In rare instances, like during full moons, that small road allows humans to venture in shallow parts of the Astral World.
The road of Dragons is contained at all times by the Tree, but the Beherit can open a gate to it. Correct me if I'm wrong, but if both the roads are related or go to a similar place, does this have implications on whether the God Hand and IOE, and nature magic and astral creatures like trolls might be related after all through this World Tree? This is a bit unclear to me.
Well the beherit can open a gate to the Abyss. It opens "the road of dragons", basically. Keep in mind this is all metaphorical. Both roads lead to the Astral World, but not to the same parts of it. As a reminder, the Astral World is layered, it has various depths and it changes drastically depending on how deep you go in it. Flora and Schierke explain this in volume 24. So very different territories (like the Qliphoth or Flora's domain) are found in very different layers, and the same goes for the kind of creatures that inhabit it. The road of elves is benign, but the road of dragons is very dangerous. The easiest example to understand is that elves and trolls are astral creatures, just like the Lady of the Depths and the Lord of the Rotting Roots. But they're not at all alike, neither in power nor in essence. The same goes for the God Hand, whose members are former humans who have become pure spiritual beings. They all exist in the Astral World, but in very different places. And the God Hand and the Idea of Evil aren't related to elemental magic.
I wonder if a character could use the world tree to take a direct road to the vortex of souls and confront the Idea of Evil. It sounds like an unnecessarily big move to me, but perhaps if they could do something to it (even just distract it, say by a group of powerful magic users), the God hand might be at a disadvantage or less powerful.
It's possible in theory, although you'd have to be sure you could keep existing at that depth, which isn't guaranteed. Like you said, I also think it's an unnecessarily big move when just facing a member of the God Hand is already a massive undertaking in and of itself. Finally, the Idea of Evil exists on such a different level that I'm not sure what one would do if they were to come to It.
Now i wonder, if this "road of dragons" really connects the physical world with the depths of the astral world, even with the vortex of souls, maybe if every forest of spiritual trees is destroyed the vortex itself will be brought in the physical world, or at least very near 
An important question right now is how the World Spiral Tree is tied to the Abyss. Because Ganishka went down to the Abyss when he became Shiva, and the many faces and confusion he displayed afterward seemed to imply that he had tapped into the Vortex of Souls. We don't have the answer yet, but it's something to look out for. That being said, I don't think the Vortex of Souls will just appear near Falconia and swallow everyone or something that crude. But maybe all those who die will be unescapably absorbed by it through the World Spiral Tree.
Also Ged mentioned that the Beherit can open the road of dragons. Maybe that's the same "road" Griffith descended in vol 13 when he perceived the deaths of his comrades and he reached the Abyss.
Keep in mind those "roads" are metaphors and refer to phenomenons we were already aware of. Like a candidate for apostlehood being brought to the God Hand's lair.
As for Griffith when he became Femto, I'm not quite sure, given that he went down to where the Idea of Evil resides.
Griffith and Aaz, you are both probably right in regards to your reply to my comment. I think it would be cool if the party split up with various members in charge of somehow restoring the forests... but real world time is an enemy. It would extend the story a lot.
I think finding another way to get the World Tree to disappear would be more simple than just restoring the forests, which would presumably take hundreds of years.
I wonder if the elves have a way for humans to travel the elf road branch... skip a boat ride back and all.
I have wondered the same thing ever since we saw the Moonlight Boy travel through the World Spiral Tree.
It seems the merging of the worlds has happened before, Gaeserics conquest wasn't as cut and dried as it looked, he may have followed a similar route to Griffith, with Void as his accomplice in world domination 
As I've said before, I think Griffith's life may well have been modeled after Gaiseric's. Except Gaiseric achieved his goal as a man, since he did not become a member of the God Hand and instead suffered a fate similar to that of Guts. This is nothing new to be honest.