Hi Eosin,
There are some things that I like and some things that I don't like.
I really like the "Hide in plain sight" ability, but it might be less of a terminology conflict with the Shadowdancer ability in D&D if this was called "Hide in a Crowd" - since that is what it actually amounts to.
I particularly like the focused attack ability; it is one that makes sense.
I don't like the sniper ability, since sneak attacks/death attacks from beyond 30' break a fairly fundamental rule in the games (about 30' being the maximum range for benefits from aiming) and it would also be a particularly powerful ability. One of the limiting factors of the 30' range for sneak attacks is that within 30' even the heaviest fighters can charge attack the missile using attacker. If they can sneak attack from 60' there will be many armoured foes who simply can't touch them. I would replace Sniper with Poison use.
I'm don't see why they should particularly gain the "opportunist" ability.
In terms of the critical stuff, I think it is a little too much. I would eliminate the greater critical and critical focus abilities, and move free "improved critical" feat from 4th level to 9th level. This gives one special ability per level, so there is still something to look forward to at each level of the prestige class.
I'm sorry if this seems more negative than positive, I do appreciate the work you have put in here, and I like the prestige class overall.
My suggestions would leave the class abilities looking like this
1 Sneak Attack +1d6, Death Attack
2 Uncanny Speed
3 Hide in a Crowd
4 Sneak Attack +2d6
5 Poison use
6 Opportunist
7 Sneak Attack +3d6
8 Focused Attack
9 Improved Critical
10 Sneak Attack +4d6
- one other thing - I would be inclined to require Disguise 5+ or something... the basis for the hide in plain sight/getting close to someone bit. Plus its an otherwise sub-optimal skill which wouldn't normally be taken, so a bit of sacrifice has to be made to reach the class.
Cheers