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.
Sniper
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).
Yes or No, depending on how you meant the question.
Sneak attack is a once per turn feature, so you may not use it on both attacks. However, since sneak attack affects one creature you hit with an attack if you have advantage on the attack roll, while using a finesse or a ranged weapon, dual wielding is a decent option, as you have two chances to hit and apply that extra sneak attack damage.
Note: You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll.
Best Answer
It's possible, but there is a couple of DM approvals required to kick this off
Let's start at the beginning, you've got a familiar that wants to be 'on' your character as a means of providing 'safer' Help action economy.
Mounting a player character (DM approval)
Generally, two characters can not end their turn in the same space. The question of mounting another creature then comes into play. This is the first step in a DM determining whether or not something is feasible. While RAW it's possible, a DM still needs to determine if a spider can actually mount a humanoid. It's not unreasonable to say that a spider does not mount a humanoid and that the proposed combo isn't viable.
But let's say they do.
Squishy Familiars and Action timing
First off, find familiar doesn't RAW allow the caster of the spell to control the familiar on their turn. In fact, the familiar needs it's own initiative (PHB, 240):
Again, the DM will need to houserule to allow the timing of this to always work. Of course, if giving it's own initiative and they're after the Rogue, they can always hold their action to Help as the Rogue closes in.
But let's say this all works out in terms of initiative order. At this point, you still have a very squishy little spider. It's easily killed via AOE attacks or most direct attacks due to it's low AC and HP.
There's still the option of not using Help for advantage and Sneak Attack, but just being within 5' as you've noted. Then the timing is less important except for it being on you at the start of combat.
And there remains a cost to bring it back the next combat. If the caster isn't a ritual caster (like the proposed Arcane Trickster), they're going to need to burn a spell slot and the required materials. It's not a bad trade-off for the increased damage output with Sneak Attack, but it is a cost that's higher than a caster with Ritual Casting.
Being an enemy (DM Approval)
If not using the Help action, then all the creature is is a spider. It's not involved in the combat or the encounter in anyway other than simply being present. If it's not actively doing something, it's a bit of a stretch that someone would consider it an enemy.
What's good for the goose...
Finally, we've got a player using a semi-reasonable tactic. The tactic is more commonly used with the owl's flyby and trusting that a creature won't target it after, but this is very similar to that. It's a big part of having a familiar and isn't necessarily bad.
But if it's always working, I'd fully expect the DM to set up monsters who also use this. The biggest thing that I as a DM always try and work with is allowing players freedom to come up with cool stuff, but if it's borderline and used too often, then i'll have a talk with them.
Owl Fly-By and my experience
When I had my wizard (before he died :( ) I used the fly-by owl to grant Help. I did it more often for my friends then for myself, but I wasn't the class that would gain a real benefit like a rogue (I did generally give the Help to the Rogue.) The DM was skeptical, but understood the mechanic. And at times they directly targeted the owl, but at other times did not. It was really based on the monsters, their intelligence, and the threat levels. But a monster realizing that this thing is making another creature really hurt them, they're gonna get rid of that thing.