At the end,they reach "Tomorrow", the ideal, peaceful place Guy talked about. But who's to say that the shifting continental plates will end? Guy may not be smart enough to determine when they'll stop (for all his brains, he couldn't figure out where the sun went when fog obscured the view), and we know that the continents never ever stop moving to this day! Who's to say they're safe again?
At least now, they have room to maneuver and the confidence in their own resourcefulness to handle it if it comes up.
Yeah, I feel like it's more about the journey and the lessons learned than the actual destination. No doubt they'll still run into conflict and strife in some form or another, but when they inevitably die, whether it's one day or sixty years after the end of the movie, they'll have lived lives full of ingenuity, courage, and ambition. So much better than living and dying huddled in fear in the dark.
It seems like the split was complete so it's likely they're out of danger and the place they were previously on was simply undermined by the splitting plates. They should be safe now.
How did Grug recognize Douglas? Thunk found him while they were all separated and he fell off the tree while Grug was still making his way to them.
Douglas fell off the tree with the seashell still in his mouth. Said seashell (along with the ones Guy gave to everyone else) were significant enough to be easily recognizable. When Grug saw Douglas, little D still had the seashell in his mouth and was clearly crying. As brainless as Grug may have been, it can't have been that difficult to put two and two together.