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.
Sneak Attack?
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.
I strike a flat-footed opponent thrice, twice with my main-hand and once with my off-hand
then, before my next turn, I also deal one strike to an opponent streaking past (AoO)
In that situation, which of my attacks benefit from Sneak Attack damages ?
in 3.5e, it would have been all, that does still apply ?
No, firstly, there is no "flat footed" in 5e, so inorder to get sneak attack damage you either need an ally adjacent to your enemy or advantage on your attack roll. For the sake of this question we will assume you have one of those.
Second, there isn't really an easy viable way for a rogue to hit three times, but let's pretend you multiclass rogue and fighter and have enough levels to get 2 attacks plus an off hand bonus attack.
As you said in your question, sneak attack only happens once per turn, so only 1 of those three hits would get sneak attack damage applied to your damage. Presumably the first one.
only one attack per opponent ? (once for my regular turn and once for
my opportunity turn)
So this question here is a bit misleading. You can use sneak attack damage once per turn, not once per round. Your opportunity attack happens when it is not your turn, so you can apply sneak attack damage. But only because your turn from your first hits are over.
only one attack at all ? (opponent of my choice, possibly wasted)
Only one attack while it is still your turn.
only the first attack ?
You can in theory decide to not apply your sneak attack damage to your first attack, but you can't do that if you already know the results of your second and third attack.
Best Answer
You can sneak attack whenever you are eligible to.
d20SRD's opinion on the topic:
There are some additional restrictions (immunity to critical hits or fortification, miss chance or concealment, improved uncanny dodge), but those are always a target's ability, lacking which you can sneak attack many times in a round, if the target is denied its Dexterity bonus to AC during each attack. Of course, if the target is denied Dexterity bonus to AC only against first attack (as is the case with striking from under invisibility, for example), only the first attack would be sneak attack.
As for your second question - no, it is not normal (as in "to my knowledge average gaming group does not use this houserule"), and no, it will just nerf the poor rogue, who cannot really brag about excessive combat ability in the first place.