While the details are somewhat glossed over, it's rather clear that the Telmarine conquest of Narnia basically amounted to a genocide. Although scattered remnants of the Narnian fantasy creatures and talking animals still remain, they have been much decimated and driven into the barren wilderness, and many/most Telmarines no longer even believe they ever existed.
Although the book doesn't specify (and indeed seems to forget about) the fate of Miraz's wife and infant son, it's fairly easy to conclude that if the nobles' stealth assassination of Miraz had been followed up by victory and their seizure of power, they'd have murdered Caspian's baby cousin sooner or later. Possibly even immediately, assuming the one of their number who took the throne already had a male heir of his own.
The Pevensies and Trumpkin get lost trying to get to Caspian's camp because the landscape has changed so dramatically, with rivers no longer running the courses they used to. As noted, it is entirely plausible that natural erosion over the course of hundreds of years is responsible for this, but there is another possibility: the absence of beavers. Nikabrik notes that the White Witch eliminated the beavers of Narnia hundreds of years ago, so that is centuries without dams stemming the natural flow of rivers and streams, and that undoubtedly changed the landscape.
The River God's long dormancy may have also had something to do with it, if we presume that some of Narnia's pre-conquest watershed was shaped by his preferences rather than Nature's.
Nikabrik, the Hag and the Were-Wolf elect to resurrect Jadis the White Witch to destroy their enemies, the Telmarines. They would have succeeded, except for the last minute arrival of Peter, Edmund and Trumpkin. So this begs the question: in the hundreds of years since the Pevensies disappeared and the Telmarines invaded, why hasn't anyone else attempted this, especially as the Hag relates how much the Witch was revered and missed, at least by the evil creatures? Well, for one thing, one would have to know the proper ceremonies, and for another, said ceremonies would most likely have to be performed in one of the magical places of power in Narnia that Master Cornelius alluded to when talking about where Susan's Horn of Summoning would be most effective. Using this info as a basis, it is not totally unreasonable to assume that resurrecting an evil witch would require one to perform the rites in a place where Evil emanations would be strong, like the Stone Table where Aslan was executed. During a major meeting of Caspian's army, it is revealed that most (if not all) Old Narnias have never even heard of Aslan's How (the mound covering the Table), let alone where it is. That ignorance would go for the evil Narnias as well.
Bordering on Fridge Horror, perhaps, but... remember how Trufflehunter the Badger constantly kept harping on how "beasts never change- they hold on"? On the face of it, this is merely a testimonial to the steadfast loyalty of animals, but for one who has read The Magician's Nephew and/or The Last Battle, that statement has a much darker meaning behind it. In "Magician", Aslan, after creating Narnia, warns the newly birthed beasts that if they misbehave, they would lose the power of speech and become Dumb again. We see this actually happen to the traitorous cat Ginger in "Battle". This is a primal fear that has been ingrained into the Talking Beasts of Narnia ever since the dawn of recorded time, and even the creatures in "Battle", who haven't seen Aslan in centuries, remember being taught this warning at the earliest of ages, and have lived in terror of this threat all their lives. Not to take anything away from the inherent goodness and genuine heroic fortitude of Trufflehunter, but the fear of suddenly and permanently losing one's mind will certainly affect your patterns of behavior.