I'd argue they can see "oh, I'm paralysed", when they fail the save.

In other cases, it opens up a hell of a can of metagaming. You could argue a trained Assassin might tell, but even then, it comes down to a "oh, my character knew this one dude, who once witnessed an assassination, so now I can tell that you are one as well". In combat, it behaves exactly like a sneak attack. I'd say it looks exactly like a sneak attack.

Perfect and Flawless mythals usually drop from epic bosses, you won't find them dropping from the regular mobs. Divine never drops outside of DM events. Beholders can be found either in the Underdark or by going to the Udos ruins cave in Forest of Despair: North East (a cave in the same area as the portal node).

As for those broken areas, which ones do you mean? They may be empty for plot reasons and for those with broken roads they may just never have been expanded on (Ostland is an example of this, but ICly they used to be explained as roadblocks by the local soldiery).

Wow, even flawless is behind bosses? That's kinda weird, perfect and divine I could understand. Question, what do you mean by -epic- bosses, how can I tell the difference between that and a regular one?

A sign said a swamp was northwest from Cordor, but there was only a west exit and then no north, so I headed west for ages (I think) trying to go north and found a bunch of huuuuuge open green fields (didn't know maps could be that large) with nothing in them and had a green field in the sky, it was really weird, then I found an empty lighthouse even further west (I think).

Also, because I found this funny, 1 hour after writing the above post, I suddenly got 3 greater crystals in a row from one spawn despite only ever finding one before this, definitely some kind of RNG intervention.