I love the class ... cut to constructive criticism.
1: Assassin attack finesse is in dire need of errata. +d6 damage with either single handed weapons or assassin weapons (read: garottes). Otherwise you simply buy Weapon Proficiency: Fullblade or Executioner's Axe. And the power curve just breaks. Hell, a maul will do it. (Oh, and Kukri Lunge becomes pointless)
That would be like a ranger (specifically a ranger with an animal companion) wielding the same weapon. Getting that +d6 per tier against their quarry.
A maul is not brutal 1, thus kukri charge still has benefits compared to a maul. Also, you would still ned a feat to get a maul (at which point you would get the mordenkrand instead which would give you the brutal 1 property as well).
It does ignore the idea of the class basically, but it makes a different build that more or less ignores the guild at-wills and is similar to the old Greatsword wielding rogues of old. However, there are already classes that could be quite similar (just trading the one shot massive damage encounter power for several encounter powers, etc). It would be a slayer with d6's instead of a static boost to damage basically.
2: There really needs to be a dagger at will. Or two. I mean. Seriously? Kukri and javelin but no dagger? Which means that one of the archetypal assassin weapons is among the worst weapons they can choose. (For melee, rapier or longsword beats it hands down. And at range if you're a ranged assassin go shuriken)
Well, the kukri is bassically an assassin's dagger. Since the dagger is already the rogue's best friend, they probably wanted to differentiate the assassin from the rogue.
3: Unarmed throw. This should be against a NAD. Ref or Fort - or a choice. AC makes little sense. Likewise the Bolas (I don't care which one- there's an argument for the bolas to hit the worse of the two).
With the Bola attack, you get +3 as a proficiency bonus and it does immobilize as an at-will attack, so AC isn't as bad. The unarmed however is pretty bad, as it is worse than the bola attack and while it technically says you gain proficiency with unarmed attacks, it does not say what that means. Missing out on a +2 or +3 bonus on the attack the throw is very poor. It should probably address what being proficient in unarmed means (either give it the monk's unarmed attack, or give it it's own version).
4: There are some odd features of the garotting rules. Like the ability to use one for an opportunity attack while having it round someone else's throat. (The trailing end of wire, I suppose?) Several possible solutions including making you only need one hand to sustain (so you can draw a normal weapon in your other hand for MBAs).
The rule on the grabbed condition are that maintaining this condition on the creatur occupies whatever appendage, object, or effect the grabber used to initiate the grab. So, in the case of the garotte, both hands and the weapon are being used. If proficiency in unarmed attacks allow the use of legs/kicks that would allow for opportunity attacks, otherwise outside of a DM houseruling, you would be pretty much unable to attack without ending the grab.
5: Unseen Spearhead. This appears to be a newbie trap at heroic when compared to Garrotte Strangle. If you don't keep the target grabbed for the entire round then it does nothing. If we ignore Vicious Greatspears for the moment then a crit does only around twice the damage of a normal attack anywhere in heroic - so assuming both hit and no escape, the garotte strangle is about on par with damage after the first round of sustaining. But this doesn't take into account that the garrote already did damage, is more accurate, and harder to escape from. You need 2 rounds before overtaking the garrote.
First of all, action point.
Secondly, based on the rules on grabbing, the spear you use to grab them (which, can't be a great spear. If it was any weapon in the spear group it wouldn't have to include javelin, which is in the spear group anyway.) can't be used to attack. So, any one handed weapon can be used on the coup de gras while you have the target grabbed with the spear. And, this is really only useful once per encounter when you can use an action point and your encounter power to pump a lot of maximized damage into a single target. Otherwise, the garotte attack is more useful. However, you can have both (especially as you level and get more). And with a ki-focus and quick change, there is little cost involved in having many weapons on you.
6: Without superior cover, total concealment, or some form of invisibility, the assassin can't hide. Which means he's hosed by certain styles of DMing (and if I see another rose bowl I'll be annoyed). Possible synergy with the missing dagger power? The dagger doesn't do extra damage, but the small weapon does allow the assassin a distraction to hide in regular cover that turn?
There are some utility powers that address that issue, and being trained in stealth allows the assassin to grab stealth skill powers as utilities that can also make it easier to hide. There is also the bluff->hide technique (only usable once per encounter though.) Taking the cuthroat feat at least makes it a minor action. There are a number of ways to deal with the problem. If it's outside, it may be more difficult, but inside of buildings there will likely be corners to start hidden behind and then using stuff like silent stalker to enter the room and remain hidden long enough to bring one of the attacks.
7: Silent Stalker. Does this automatically unhide you at the end of your turn? Or can you use it to "sling shot" - silent stalk to the guard and then take your second action to run past them to the next spot of cover. (I'd allow the sling shot, myself, but can see arguments both ways).
They should probably clarify the wording, but presumably if you are in a position to maintain your stealth at the end of your turn you wouldn't lose your stealth. [The power is designed to set up an attack, and thus assumes you left cover as part of the move]. Then again it may be a built in draw back, you get to be hidden until the end of your turn no matter where you are or what you do (leaving cover, attacking, etc) but you are no longer hidden at the end or your turn no matter what you do.
8: Was the intent for any or all of precision darts, bolas takedown, and unarmed throw to gain the bonus damage from Attack Finesse. And if they do do they also gain the enhancement bonus?
I'm not sure about it. The extra damage isn't worded like sneak attack (it would apply to maintained garotte strangle since it isn't tied to a hit like the other extra damage effects are). It itsn't worded like enhancement bonuses, so it should apply. However, it isn't weapon damage, so it wouldn't allow enhancement bonuses to apply [that I do know, there were questions involving that in the past in the case of sneak attack with a fixed damage power].