Roshan's name means "Light at Dawn" in Hindi and it is therefore fitting that he leads the three main characters out of their dark pasts and is a major factor of them becoming True Companions.
One might wonder why Manny would even think about helping Sid return Roshan to his Father? Humans are the reason he lost his first family and it's clear he is hurting throughout the film. But since he knows what it's like to lose his family, he probably wouldn't wish that on his worst enemies, even humans. Furthermore, by spending time with Roshan, Manny learns to move on and even forgives the humans for what they have done where he parts ways with Roshan and his tribe on friendly terms in the end.
Knowing that Half-Peak is confirmed as being a volcano (a dormant volcano) in the novelisations, it makes perfect sense why there's a lava flow underneath the ice. The lava flow laid dormant there after the last time the volcano erupted.
A lot of Manny's lines from the first act of the first movie make a lot more sense once you know his backstory.
Manny:[to the rhinos] I don't like animals that kill for pleasure.
Soto sometimes seems to be trying to get Diego killed. Considering that they're pack animals, Soto saw Diego as a potential challenger to his position as leader of the pack and was trying to get rid of him before that happened. It came true, though not in the way Soto expected.
Diego's body isn't bloodied for one simple reason: most of his injuries are internal (and most of them can be attributed to when Soto struck him into the boulder, no doubt causing a concussion and myriads of broken bones scattered throughout his body) and it's possible that where Soto gashed him he deliberately laid on top of those wounds so that the other members of the herd wouldn't be unduly worried.
Diego's attempt at turning a sabre pawprint into a human footprint in the first film was laughably bad, but probably made sense from his standpoint. He drew a bare foot rather than a shoeprint because none of the animals completely understand when or why humans wear clothes, and he gave it exactly four toes because that's the amount of digits that most animals (including saber-tooths) have in the film.
Manny is heading towards the freezing wasteland and it's implied that this wasn't too long after his family was killed, apparently trying to commit suicide after mourning the loss of his family and just wants to die in order to be with them again.
Yes and when Sid first runs into him he's where? Right at the edge of a cliff about to jump.
The cliff would have been a Bungled Suicide had Manny tried it, considering that he fell off it with Sid and survived.
The fact that Manny's family were killed recently makes a lot of the film's scenes much darker. Particularly the scene where he hands Roshan to his father. The father sticks a spear (the same weapon used to kill Manny's family) right in Manny's face, and Manny shoots him such a stony Death Glare that you have to wonder whether he was physically holding himself back from maiming the guy with it out of mindless revenge.
In the cave painting scene, we see that Manny's mate never stood a chance. Surrounded by speared humans, she could only shelter their calf helplessly which made her unable to lash out with her feet, trunk or sheer bulk. Plus, elephant/mammoth necks aren't flexible enough to let them look straight up, which the human rock-wielders on the clifftop clearly knew.
The official novelization has Soto point out to Diego that Half-Peak is a volcano before instructing him to bring back the baby or Soto would kill Diego himself as retribution. Later on in the film, Manny and Diego and Sid nearly escape with their lives after the dormant lava flow underneath Half-Peak erupts. It makes one wonder if Soto knew it was going to erupt when he went on before Diego, thus dooming him.
