The Trees fell during the First Age; the Downfall of Númenor and the Changing of the World happened towards the end of the Second Age, almost three thousand years later, and involved Sauron, not Morgoth, who'd been defeated and exiled from the world at the end of the First Age.

As for Legolas, he was a Sindarin elf born in the Third Age. He never saw the Trees, and had never been in Aman at the time of The Lord of the Rings.

The only named characters in The Lord of the Rings (other than ones mentioned in songs and legends) who had ever seen the trees were Galadriel, possibly Celeborn and Glorfindel, technically Gandalf and Saruman (provided the Istari count as the same people as the Maiar they were back in Aman), and, maybe, Tom Bombadil and Goldberry, but who knows, really, with those two. (I don't think Sauron ever was in Aman, at least during or after the time of the Trees).

So no they can use sign language to talk to someone on another planet of some other star?

Hmm. That should allow us to estimate the size of that world. The light of the trees must not be so bright as to cook everything in the vicinity; just make it nice and balmy. But, on the opposite side of the world, there must still be enough light to see. Having the occasional photon bounce back would eventually be enough to make out a static scene, but, apparently, it's possible to see things happening in real time, yes?

Does flat mean that we are talking about something like a simple disc here, or just that a beam of light travels parallel to the ground? The latter would imply a rather strange geometry, which I can't wrap my mind around. It would make more sense, though, as, obviously, we couldn't assume that light intensity diminishes with the {ETA:] square of the distance.

they could actually see things that were on the opposite side of the world.

But there's mountains and slopes and all.