Assuming you can't get a theme, here's my suggestion for a damage-focused crossbow sniper:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 14
Drow, Thief, Dread Fang
Background: Occupation - Criminal (+2 to Stealth)
FINAL ABILITY SCORES
Str 9, Con 12, Dex 24, Int 11, Wis 20, Cha 11.
STARTING ABILITY SCORES
Str 8, Con 11, Dex 18, Int 10, Wis 14, Cha 10.
AC: 29 Fort: 21 Reflex: 30 Will: 25
HP: 89 Surges: 7 Surge Value: 22
TRAINED SKILLS
Stealth +23, Thievery +19, Streetwise +12, Dungeoneering +17, Athletics +11, Acrobatics +19, Nature +17, Perception +17
UNTRAINED SKILLS
Arcana +7, Bluff +7, Diplomacy +7, Endurance +8, Heal +12, History +7, Insight +12, Intimidate +9, Religion +7
FEATS
Level 1: Ruthless Hunter
Level 2: Primal Sharpshooter
Level 4: Two-Fisted Shooter
Level 6: Backstabber
Level 8: Crossbow Expertise
Level 10: Weapon Focus (Crossbow) (retrained to Lasting Frost at Level 11)
Level 11: Primal Eye
Level 12: Wintertouched
Level 14: Silvery Glow
POWERS
Lolthtouched: Cloud of Darkness
Primal Sharpshooter: Grappling Spirits
Thief utility 1: Tactical Trick
Thief utility 1: Escape Artist's Trick
Thief utility 2: Fleeting Ghost
Thief utility 4: Sneak's Trick
Thief utility 6: Chameleon
Thief utility 7: Ambush Trick
Thief utility 10: Counter-Step
ITEMS
Frost Hand Crossbow +3, Feytouched Drowmesh +3, Bracers of the Perfect Shot (paragon tier), Assassin's Cloak +3, Eagle Eye Goggles (heroic tier), Frost Hand Crossbow +2, Potion of Healing (heroic tier)
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I calculate DPR with to-hit chances, so my numbers will be much lower than yours.
Ranged basic attack: +23 v. AC, 1d8+26 cold damage
Combat advantage from 2 sources: wintertouched/lasting frost with a 1-(28-23)/20=75% chance and hiding based on invisibility or sneak's trick (any source of concealment will do)
When you start your turn hidden (for extra juice there, take persistent tail as your level 10 utility), you deal 4d8 as your sneak attack damage.
Tohit: (1-(28-(23+2))/20) = 85% hit rate, with backstabber means you generally won't miss.
DPR: .8*(4.5+26+4.5*4+5)+.05*(8+26+8*4+5+3.5*3+.85*(4.5+26+4.5*4+5)=49.14 DPR, all encounter long every encounter. Backstab to be used to turn misses into hits and you're a walking damage machine. AP of course gets you a second verse same as the first, so that's 100 damage on the first round. at 136 average HP for level 14, the slowed enemy (grappling spirits) gets a round to croak for help before you finish them in the second round. No dailies in the class which makes for an incredibly long-enduring character.
Free reroll on stealth checks from your neckslot with a standard action invis from your armor gives you all the stealth you really need. Especially with sneak's trick allowing remarkable stealth in places that would not otherwise be considered venues for sneaking. Average non-trained perception is +10 at 14, so a +23 with a reroll and an II to become re-hidden should provide for sufficient stealth.
For a nova-based assassin... I'll look at the executioner with big, sad, eyes... and go to the Ranger. Their damage simply cannot be beat. I'd personally choose a wind-rider, but that's because I find pure-damage frostcheese excruciatingly boring. The darkstrider gets you your hidden damage boost. Spending feats on skills is usually contraindicated. See here for a goodenough archer that just spams the damage.
If you choose to not go with thief (cunning sneak is slightly less good than sneak's trick, depending) or ranger, then executioner isn't a bad choice. You're trading damage for flavour, but it's very nice flavour. If you MC warlock and take shadow dancer then it's very nice, very very very very mobile flavour. Warlocks also make good assassins MC into assassin :)
Cunning sneak is just not that worth it in actual play. While it helps in theoretical "solo stealth" missions, it's hard being a meat&potatoes assassin. Bravo's a trap. If you're going PH1 rogue/ranger, focus on minor action attacks, multi-attacks, and frostcheese. They'll get you the most damage with the least tradeoffs. Darkstrider becomes better there, because 3+wis damage on multi-attacks very very quickly becomes stupid.
Don't forget to calculate to-hit in your damage potential.
Good luck. Feedback will be incorporated to refine requirements and my suggestions.
I would say Y has to do it since Y does not see or know the danger involved in doing what you are suggesting. The only thing that I can see wrong here is that if Y somehow knows that what you are suggesting is 'eventually' dangerous to him.
I base this on the description of charm/dominate where it clearly states that the effected will perform any action that does not pose a threat to his own health. Literal copy of the RAW (I used the 3.5e rules here since they offer a nice example which the Pathfinder version does not) for charm person:
An affected creature never obeys suicidal or
obviously harmful orders, but a charmed
fighter, for example, might believe you if
you assured him that the only chance to
save your life is for him to hold back an
onrushing red dragon for “just a few
seconds.”
The suggestion that you made to have Y move to a certain square sure seems to be a lot less dangerous than what is described above, so if it works with charm it should work with dominate/diplomacy in my opinion.
Best Answer
Caveat: the answer to this question is going to depend heavily on your DM, and how they feel about and portray the use of social skills. Some DMs don't much care, and will allow you to use these skills with little penalty or roleplay required. Other DMs may view social skills as potentially game-breaking, or at least very important to handle correctly. If you intend to use your Intimidate often, I would recommend finding out your DM's policy on social skills, and adjusting your expectations accordingly.
That said, in general, Intimidate is meant to be a representation of your character's ability to bully or scare someone. So, to address your three points:
To "cow a target into taking some other action" generally means to frighten them into doing something other than what they had planned. For example, let's say you're fighting a group of goblins. One of them runs at you with his sword. You roll an Intimidate check, and describe the action as "I tower over him, my staff smoking from my last spell and my eyes glowing with power". A successful check might cause the goblin to attack someone else instead. Alternately, if you are particularly successful (let's say you rolled high on your Intimidate check, and in the previous round you critted a different goblin) you might scare him into losing an action or even running away.
Can you use Intimidate on NPCs? Yes, but remember: it's a representation of your ability to bully or scare people. Intimidating townsfolk might get you the information you need about the evil lich lord - but it might also cause the NPC's bigger, more powerful friend to come after you for bullying his pal. Likewise, bullying a shopkeeper once might mean he hides his best wares next time you're around, or simply closes up shop entirely to avoid you.
(2a) You didn't ask specifically, but it's the flip side to #2 so I wanted to address it. As a DM, I don't allow social skills like Intimidate or Diplomacy to work on other PCs unless you both a) roleplay it out, and b) have a very good reason for doing so. Skill checks are meant as an abstraction; when you are interacting strictly with other players, you don't need that same level of abstraction. It's very frustrating, as a player, to be forced to act a certain way because you rolled badly and your fellow player rolled well. But again, this depends on your DM and your group, and their take on social skills in the game.
Do targets become hostile out of combat, and what does it mean? They will very likely become hostile - as I mentioned, even if your initial target is too scared to fight back, his big bad friend might not be. Or, if you Intimidate a member of the criminal underworld, say, he might act on his fear by drawing a weapon on you, forcing you to fight (and spend resources) when you might otherwise have not needed to. And 'hostile' doesn't always have to mean 'immediately attacking'. A hostile bar wench, for example, might get revenge by poisoning your drink, or by spreading vicious rumors that bring the city guard down on you. A hostile merchant might raise his prices, or avoid selling to you. And so on.
Intimidate, like the other social skills, can be very powerful when used correctly. The key is to ask yourself whether bullying or scaring someone is the best way to get what you want out of a situation. If the penalties are worth the payoff, then go for it.