Does a Barbarian need to start out adjacent to his ultimate target to use Pressing Strike?

Question

1. Suppose a Barbarian (with a reach 1 weapon) isn't adjacent to any enemy, but there is one 1 or 2 squares away. Can he use Pressing Strike to shift adjacent to that enemy and attack all in one action?
2. Suppose a Barbarian is adjacent to enemy A, but really wants to attack enemy B who isn't adjacent. Can he use Pressing Strike to shift away from A and get to B?

Or is Pressing Strike only usable to circle around/barge past the enemy you want to attack? Basically, when are you required to declare the ultimate target? What if the target shifts or teleports away as an immediate action while you're in the process of shifting -- can you pick a different target or does the action end?

Pressing Strike
At-Will Standard Action
Melee Weapon
Target: One Creature
Keywords: Primal, Weapon
Effect: Before the attack, you shift 2 squares. You can move through an
enemy's space during the shift but you can't end there.
Attack: Strength vs. AC
Hit: 1[W] + Str modifier damage, and you push the target 1 square. If you are
raging, the attack deals 1d6 extra damage.

Answer 1

Based on what you posted the following is true:

You must start the attack in range for the attack

However the online compendium has a different version. It reads

Pressing Strike
At-Will Standard Action
Melee Weapon
Keywords: Primal, Weapon
Effect: Before the attack, you shift 2 squares. You can move through an
enemy's space during the shift but you can't end there.
Target: One Creature
Attack: Strength vs. AC
Hit: 1[W] + Str modifier damage, and you push the target 1 square. If you are
raging, the attack deals 1d6 extra damage.

The different order here changes the ruling quite dramatically.

Now both of your scenarios are possible as you must only be in range after you have shifted as the target must be a valid target by the time you have shifted away.

As far as the case of the immediate action, it still depends on the trigger. If the trigger is the movement adjacent then you are free to pick a different target (you've not yet made the decision as far as the power is concerned). However, if it's an immediate interrupt on the targetting of the mob then the action is lost.

All this said, Howling strike with a charge is often a better option than pressing strike, unless you can use pressing strike to set up flanking. Flanking is better than charging, unless you are well optimized for the charge (horned helm and vanguard weapon come to mind). The bonus damage for either is the same so you should be aiming for the biggest to-hit bonus.