Regarding the absence of Kanga and Roo — Kanga was probably worried about Roo going on an adventure like this, so she stayed home with him.
And as Fall had just started Gopher would have gone into hibernation.
Pooh was once mistaken for the Skullosaurus due to the crystals distorting his figure. It makes so much sense when you realize the "growls" of the Skullosaurus were really Pooh's stomach growling from the lack of honey.
Seemingly, Owl's map is useless and no more is that present than in the sinister looking forest to which Owl marked as "Peaceful Forest". However, when the gang walk back through the forest, we see that it's much more peaceful looking than we first saw it. Maybe Owl was more Genre Savvy then we gave him credit for!
When you see how shallow the creek is that they land in after the thousand-foot drop off the gorge. If they weren't stuffed animals (and this weren't a kid's movie), they would be dead.
Regarding the gorge, take a look at the map again. The way to Skull was over the gorge, not under it. In a way, the gang can't say the skullosaurus isn't real because they didn't actually get to skull! Does this mean the skullosaurus actually exists?