1. Sam and Eric are identical twins. It was difficult for the characters to recognize which one was which that is the reason they were named Samneric. This way when a twin is called Samneric different characters won't have to identify which twin they were talking to. Sam and Eric ended up one entity.
Sam and Eric were characterized as great, ordinary individuals who are not sufficiently able to resist the draws of evil. Indeed, even to the degree of fighting each other.
2. Ralph is worried since all the young men are relying on him so he is relived when Jack needs to lead the pack for a minute since this implies Ralph can loosen up a little bit. This is foreshadowing to when Jack totally takes control and begins his very own reign putting everybody's lives at risk including Ralph's.
If I word it differently: (not changing the words, just another answer or possible way to say it)
He's getting worried from leading an entire gathering of young men. He realizes that Jack needed to be the pioneer in the first spot, this minute will presumably anticipate times like this where his pioneer responsibility will be utilized by another.
3. I think that the brute they saw was one of them and the monster being "just us" alludes to them as they have moved toward becoming. They were once human however in being caught for such a long time alone in the wild they have turned into the mammoth. They are never again simply little young men yet have returned to becoming the wild monster within. They have given in to their very own animal nature for their survival.
If I word it differently: (not changing the words, just another answer or possible way to say it)
Simon demonstrates a different perspective then the different young men, he speaks to innocence symbolically, he sees that the Brute is truly them, they're continually arguing, they can't oversee themselves, they might even fight to the passing just to determine a pioneer. The genuine mammoth is their own ignorance. Simon sees that.