Coming Out From Behind The Wall
Maybe. Here's the thing. If the Rogue has to come into the open to do this, he's no longer hiding (you can't hide in the open). No spot check is needed to see the Rogue if he just walks out into clear sight of the Ogre, he's no longer hidden. As he's no longer hidden, he'll need some other condition to allow for a Sneak Attack (flanking for example).
This is one of the reasons why Hide In Plain Sight is so good. If you had that, the Rogue could attempt to remain hidden even out in the open and this would work. Shadowdancers for example can stay hidden if they're near a shadow. That would include the shadow of the wall or the Ogre itself, making it a LOT easier to attack while hidden.
Rules Compendium Changes
The Rules Compendium (page 92) adds some extra things to Hide, if you're using it. Those address coming out from hiding a bit differently. In this case the answer would be Yes, if you can make the new Hide check as detailed below.
Move between Cover: If you’re already hiding thanks to cover or concealment, and you have at least 5 ranks in Hide, you can make a Hide check (with a penalty) to try to move across an area that doesn’t offer cover or concealment without revealing yourself. For every 5 ranks in Hide you possess, you can move up to 5 feet between one hiding place and another. For every 5 feet of open space you must cross between hiding places, you take a –5 penalty on your Hide check. Movement speed penalizes the check as normal.
Sneak up from Hiding: You can sneak up on someone after emerging from a hiding place. For every 5 feet of open space between you and the target, you take a –5 penalty on your Hide check. If your Hide check succeeds, your target doesn’t notice you until you attack or perform some other attention-grabbing action. Such a target is treated as being flat-footed with respect to you.
Yes. It's specifically mentioned in the Hide skill:
If you’ve already successfully hidden at least 10 feet from your target, you can make one ranged attack, then immediately hide again. You take a -20 penalty on your Hide check to conceal yourself after the shot.
Being aware of someone isn't enough to prevent Sneak Attack. If the spot checks fail against the new hide check, then the Rogue is hidden again and can continue to sneak attack.
Hiding and Sneak Attack
Yes, being hidden with Hide makes you effectively invisible, which normally allows Sneak Attack. Again from the Rules Compendium (p. 92):
If you’re successfully hidden with respect to another creature, that creature is flat-footed with respect to you. That creature treats you as if you were invisible (see page 76).
Only effects requiring attack rolls yield critical hits
The splash damage of an alchemist's bomb isn't increased on a critical hit because the alchemist didn't make an attack roll to hit the splashed creatures with the splash damage. The rules for Critical Hits say
When you make an attack roll and get a natural 20 (the d20 shows 20), you hit regardless of your target's Armor Class, and you have scored a "threat," meaning the hit might be a critical hit (or "crit"). To find out if it's a critical hit, you immediately make an attempt to "confirm" the critical hit—another attack roll with all the same modifiers as the attack roll you just made. If the confirmation roll also results in a hit against the target's AC, your original hit is a critical hit. (The critical roll just needs to hit to give you a crit, it doesn't need to come up 20 again.) If the confirmation roll is a miss, then your hit is just a regular hit.
A critical hit means that you roll your damage more than once, with all your usual bonuses, and add the rolls together. Unless otherwise specified, the threat range for a critical hit on an attack roll is 20, and the multiplier is ×2.
Emphasis mine. The rules for when a creature takes the action Throw Splash Weapon include
A hit deals direct hit damage to the target, and splash damage to all creatures within 5 feet of the target.
The alchemist made an attack roll to hit directly the target; the alchemist didn't make an attack roll to hit the creatures within 5 ft. of the target. Thus even on a critical hit the alchemist deals his normal splash damage (although he still deals extra damage to the target hit directly).