1. Nah, you want them to be more frail. This is an Antarctic expedition, and it would ruin the tone if your players were faced with the progenitors of the human race and went "so anyway I started blasting" rather than being over-come with awe.
2. I'm currently on Chapter 4 with my group. So far they liked a lot of the character interactions and the "down time" they've had in New York, but it is starting to feel very "When are they going to get to the fireworks factory" and a lot of them have lost sight of that Antarctic as a goal. Some advice I have for the early chapters is the play up the press presence surrounding the expedition - it got to a point where my players were more concerned about making a good impression than loading the boat, and it made them consider their actions more when they ran the risk of loosing sponsors from bad press.
Use the early chapters as an opportunity to introduce the personalities of the NPC crew members. You're going to spend lots of time with these chaps, and it adds to the horror to have a cast of beloved characters picked off along side the PCS. Plus, your players are probably going to have to take over one of the crew when their character dies so it's good to let them know how these people work.
Don't be afraid to remind them of the stakes, even this early in the campaign. One of the investigators broke his back during the events of chapter 4, and it's made every other player realise their character's mortality.
3. The Campaign takes a darker turn after this so the Crossing the Line is important for adding one last gasp of levity before some of the darker moments set in. If the Investigators don't take part, it also makes the hunt for the saboteur more challenging if the crew comes to resent you for revoking their tradition.
There's a great resource of drivethru (https://www.drivethrurpg.com/product/251994/Cinematic-Environs-Arctic-Lands-Call-of-Cthulhu-Edition?manufacturers_id=2) for CoC in extreme conditions, and one of their new rules is Sanity Loss from Isolation on a ship, in mountains, a desert etc. A homebrew rule I've made is that you're immune to isolation insanity loss if you take part in a leisure activity, and large parties like the Crossing ceremony give a five day leniency period on the sanity rolls. Its worth mechanising things like this to make them feel more relevant.