[sblock=OOC ]I misread, there are indeed kobol on the wall. Edited my previous post. Still want to get to I15. I expect the boulder won't go forward and then make a curve just to squish me. Well, I hope.
Note on stealth as I understand it (and these are IMO);
1 - AFAIK, comrades only provide cover for ranged attack purpose and can't be used for stealth. They don't even provide cover for area or close blast attacks, much less for stealth, IMO.
The fact that the cover works only one way when it is obvious that your ally obstruct your view as much as he obstructs your enemy makes it obvious IMO that this is simply a conditional modifier that apply only to a precise situation (enemy makes a ranged attack against you) and nothing else.
2 - As long as the enemy as LOS and you don't have cover, perception automatically succeeds.
In the curent room, the opportunity for covers are pretty damn slim and I think that it's on purpose. Do note how the previous map had those convenient coffin in the middle of the room, guaranteeing that you could have cover and use steaklth against anyone. That's one stealth firendly room followed by one that isn't. Not every maps are equally stealth friendly, though if you keep fighting in open fields you can start suspecting the DM is out to screw you.
3 - I don't think the maths assumes that rogues gets sneak attack every round. I think that it assumes you get sneak attack every 2 round at heroic level on average. That's why a rogue has a 2/1 advantage in 'striker' damage. If a rogue manages to get a sneak attack opportunity every 2 round, he is doing just as well as a warlock or ranger. If he gets an opportunity 3 rounds out of 4, he is outpacing them by 50% on the extra damage.
At paragon level, the ratio change from 2/1 to 3/2, IMO this is because it becomes much easier as you level up to get combat advantage through conditions inflicted by your comrades or yourself and at that point and you become less dependant on stealth. As a paragon, if you get sneak attacks opportunities 2 rounds out of 3 you are doing as well as other strikers.
*Note that with flanking bonus, I expect that a rogue willing to get in melee a lot can maintain a higher than 50%/rounds sneak attack opportunity but he does so in exchange of taking greater risks whih is its own balance.[/sblock]