Hiding behind a wall
If you have partial cover, you may use Stealth. If you have total cover (wall is taller than you, or floor-to-ceiling, or whatever), you don’t even need Stealth. So you may hide behind a wall.
Readying an attack
You may then ready an attack for when he enters a square next to you, since an Attack Action may be done as a Standard and you are allowed to ready those. Your DM may require that you specify which square; the rules leave it up to the DM how specific you have to be. In any event, if this readied action is triggered, however specific your DM requires it to be, you then attack the target.
The most recent errata gives you Total Concealment until after you make an attack. So what does that give you? Well, it means he doesn’t know what square you’re in and takes a 50% miss chance even if he guesses the right square. Does that let you Sneak Attack?
No. It does not. Sneaking does not actually let you Sneak Attack.
Commentary on this errata seems to indicate that the point of the errata was to allow rogues to Sneak Attack while, ya know, sneaking, but that doesn’t actually seem borne out by the rules.
So the rules don’t actually give you a Sneak Attack, even though you are considered “hidden” for that first attack (the one you readied). If your DM is sane, he’ll allow sneaking to trigger Sneak Attacks, because it’s ridiculous that the rules don’t allow it. Paizo’s lead dev has even stated (if you go digging through the forums) that it’s supposed to work; why he doesn’t just fix the rules is anyone’s guess, but there it is. Ultimately, this is something you’ll have to ask your DM about.
Does this sound flawed...or brilliant?
Considering that it doesn’t work, quite flawed, but even if we assume it’s allowed to work, it’s still pretty poor damage most of the time. Rogues tend to have very little combat presence at mid-to-high levels if they can’t get full attacks with Two-Weapon Fighting to multiply their Sneak Attack damage. At low levels, it could be a useful trick.
Of course, with the various ways that Pathfinder has shafted rogues’ ability to Sneak Attack multiple times in a round, it may be the best you can hope for. Unfortunately, it’s not very good.
No, this isn't possible. To attack with Two Weapon Fighting, you need to take the Attack action, not the Ready action.
When you take the Attack action and attack with a light
melee weapon that you’re holding in one hand, you can
use a bonus action to attack with a different light melee
weapon that you’re holding in the other hand.
The fact that you're using the Ready action to ready an attack is sadly irrelevant - if you want to use two-weapon fighting, you have to attack first.
You could do this if you were under the effects of Haste, using the Haste action to attack and triggering two-weapon fighting, then using your action to Ready. Of course, at that point, you wouldn't actually need two-weapon fighting anyway.
You can take the Ready action after you use a bonus action, sure. What really matters though is how you get your bonus action. If you just have a feature that lets you take your bonus action without any conditions then you can go ahead and do that. But some bonus actions have conditions that must be met in order for you to take them, like the one from wielding two weapons, as I will detail below.
Taking the Ready action does not count as a separate turn. What happens is, you decide on a triggering event, something you can perceive that you will react to in the future before your next turn. Then you pick the way you want to react to it. For example, you can take the Ready action to respond to an enemy approaching your companion and respond by throwing your dagger at him. Note: In order to actually respond to the trigger you described, you must use your reaction for the turn.
Two Weapon Fighting(PHB Page 195):
So from here we can see that, in order to get your off-hand attack with your bonus action, you have to have already taken the Attack action. This eats up your action for the turn, so you will not be able to take the Ready action like this, unfortunately.
What this ultimately means is that you wouldn't have a way to throw a dagger as a bonus action (unless you found a way in-game, like if your DM gave you a magic item). If you could do it though, you would be able to make both attacks with Sneak Attack, as RogueBurger points out in his answer with a reference to a Jeremy Crawford tweet.