The basic math concept is detailed in the Behind the Curtain section.  *I* think the requirements fine because I wanted chases to be short, exciting bursts in the narrative.  So a for a group of 4 players getting to 6 and even 4 Momentum in 3 rounds is going to require a bit of sacrifice.  But if you have a larger or experienced group, or one of the players is working the system really well (and that's cool – let them be awesome now and then), there are several built in methods of ratcheting up the difficulty:

• a short, desperate chase (2 rounds to achieve Momentum)
• increase the Momentum requirement (I recommend an upper limit of 10)
• increase the Obstacle dice (since they are on odd results, you should average 1 obstacle for every 2 dice rolled), or spend danger points to turn regular Obstacles into Major Obstacles (which can cost the group Momentum)
• spend Danger Points to increase the Raise requirements from 10 to 15 (for when the opposition has some advantage over the heroes).
• if the object of the chase is a villain, just spend the influence point to escape and skip the chase altogether.

And there's no reason you have to stick with just one of these methods.  You can flip as many of these switches as you feel you need to.  But run a few basic chases first so you and your group can get the feel for them before you start turning up the heat.  The object of the chase variant is to keep the action focused on the players and to make each successive round feel more tense than the previous one.  If your players find chases frusterating, drawn out affairs against impossible odds ("You have 2 rounds to get to 10 momentum and I'm spending a danger point to make raises 15!") you are going to suck all the fun out of them.

By my reading of Momentum in the intro, "Building Momentum is a Special Opportunity that costs 1 Raise per Hero in the scene," I understood that for the 'group' to achieve 1 Momentum, all Heroes had to spend 1 Raise towards building Momentum (Succeed or fail as a group). However, in the example, Carmena spent 2 Raises to build Momentum (since Rory could not spend 1 Raise) and yet they received Momentum 2. Can you help clarify what I'm not understanding?'

I should really go and edit those to say "shared Opportunities".  Momentum works kinda like a Shared Consequence.  The size of group tells you how many Raises you need to achieve a rank in it.  It doesn't matter who spends the Raises.  If you have one player who with an advantage that turns all her dice to Raises in a scene, spend those Raises in place of the other players at the table, giving them room to do other things.  Now, ideally this is going to cost each player 1 Raise, because between Consequences, Opportunities, and Obstacles, there should be plenty of things competing for those Raises.  But as long as the number of raises is met, it doesn't matter where they come from.

For Obstacles, my assumption (and impression) is that unlike Opportunities or Consequences, Obstacles *must* be overcome by each Hero (or removed by a special Opportunity) before Momentum can be gained in the Scene? Is that true?  If not, I'm trying to understand the main difference between an Obstacle and a Consequence of "Take one Wound" or "Take one Wound and lose 1 Momentum"

No, you do not NEED to counter all Obstacles in the scene to build Momentum.  An Obstacle is a type of Consequence that must either be overcome by spending a Raise or the player takes a wound.  A MAJOR Obstacle is one that not only inflicts a Wound if you don't counter it, but also potentially costs Momentum.  I wanted to distinguish Obstacles as their own thing because an Obstacle affects ALL THE HEROES in the scene.  Different approachs lend themselves to different Consequences.  But an Obstacle is right there in the fat middle of everything and eithe needs to be overcome or it costs you.  Plus, these aren't exactly like shared Consequences.  Your GM might rule that another player could spend a Raise to help you past that barrel, or not.

With a regular Obstacle, you can spend a Raise to overcome it (jumping over a barrel) or not and take a wound (stumbling past it, bouncing off of it, etc.)

With a Major Obstacle (a cart full of barrels crossing the road in front of you), you have to spend two Raises to fully overcome it: one to avoid a wound and one not to lose Momentum. 

Faced with a Major Obstacle, you COULD save your raises, take the wound AND the loss of Momentum, and STILL spend a Raise towards building Momentum later in the round.  But then you lose as much as you gain.  If you pay attention to the math, at some point you are going to have to make a hard choice between taking wounds and building Momentum.  Losing Momentum just makes that harder.

Do all Players need to devote at least 1 Raise to a Major Obstacle during a scene for that Obstacle not to reduce Momentum for the group?

That's totally the call of the GM.  I can see there could be circumstances where I would not allow that, and others where I wound.  By the letter of the rules in CttC, I would say treat it like a Shared Consequence.  But that's a broad blanket statement that isn't meant to cover every possible scenario.

Something to consider: in CttC, fully embraces the idea that Wounds are not necessarily physical injuries.  They could be fatigue, confusion, distraction, disorientation, irritation, you name it.  A chamber pot poured on top of you does a wound unless you counter it, because its really hard to focus on your objective when you are wet, nasty and smell like piss (or worse).  So have fun with it.