I don't really think I'm overestimating her W's mobility at all; rather I think you are incorrect in your statement. The dash range is 600 but since the radius of the AOE is 250 units, her actual range on the ability is 850. Not only is that significantly longer, but it also means that she can trade with a champion like annie without annie being able to react at all (both her Q and the detonation are outside of annie's range, making a QW burst of 140+140% ap at level 2). And against someone that isn't point and click, once Leblanc has W'd, there is almost no chance that you hit her due to the ability to blink back. Trading becomes almost impossible. So I do think she actually zones pretty damn well. Which brings us right into ganks.
A jungler can come gank a Leblanc but once she hits 6 it is nearly impossible to do so due to double distortion. But even if you blow her double distortion in a gank, it is up in around 30 seconds. So unless you plan on coming mid every 30 seconds, she will continue to be able to pressure your laner with impunity. There are almost no drawbacks to her ultimate and I think this is one of the major problems with her.
Now of course the common argument is to wait for her to use distortion on your mid laner. Well, okay, first of all, if you are waiting for the assassin to use their burst on your squishy before you make your move, you're entire purpose of being there is irrelevant. Ganking a dead mid laner isn't very helpful. Now, even assuming LB can't kill your mid laner outright, all she has to do is ward the immediate bushes; if you aren't sitting in one of those, she can blink back and get to safety before you can successfully close distance on her. As long as Leblanc is careful about when she engages, she really can't be punished by a jungler. More to the point, every failed gank that a jungler attempts on a LB puts him (or her) further behind; now leblanc is setting two people behind at no extra effort or risk. Seems like a win win to me.
You mention the W to close distance and the QE for poke, but that is, in my experience, almost never used. It's a waste of mana. W in, QR is MUCH more likely with guaranteed damage of 310+140% AP, a cool down of close to 30 seconds, mana cost only for a Q and W (150 total mana), an effective range of 1300 (I realize she is dealing the damage at 700 range, but can put that 600 range in between you immediately on completion), and an instant escape making it impossible to put down more than one spell as retaliation. It also has the benefit of completely ignoring minions, making it even more difficult for things like Ahri charm (since you mentioned it) to hit. Also, no CC in the game lasts for 4 seconds, which means if it takes you longer than the duration of your hard CC to land a spell rotation, LB will be able to blink back before you're finish trading with her. And again, if her cool downs from that trade don't come up before yours do, your resources (mana and health) trades in the exchange will favor her.
I play a LOT of xerath. I'll tell you how that goes. Firstly, I'm very lucky if my stun doesn't hit a minion instead of leblanc. Secondly, the dash happens so quickly, I'll be lucky for a .85 second stun, which means she will be able to blink back before I land my W. Forget my Q all together. Now I have blown my only semblance of an escape on a trade that, in most cases, I will lose because if she maxes Q first, one hit of her Q will out damage my one point in e at 3 levels. If she is able to connect the W because my E doesn't hit, she out trades with 2 points in the skill. Remember, the 250 unit radius means she doesn't need to come right at me. For reference, Xerath's W is 200 units. The AOE on Leblanc W is bigger than that. Once she hits 6, this gets even worse when she just trades with QR and uses the W to juke. This is VERY far from a good match up and honestly just favors Leblanc.
In addition, I would like to point out that her passive stealth allows her to escape from even more; you can't hit a target that is stealthed without vision (obviously) and this can give LB the time she needs to blink back/distort away. Again, I would consider this a fair trade IF Leblancs total burst rotation wasn't available every 30 seconds. This means that your mid aners commital with a hard CC (which could very well be their ultimate) gets negated essentially by a stealth, only to have LB have her full rotation up half a minute afterwards.
To be clear, I don't think Xerath (or any dedicated mage) should be able to win this matchup. I just don't think its fair that she has the ability to kill me/ poke me out of lane with one rotation of spells every 30 seconds. And it isn't like her playstyle or methods change based on opponent; she will always out trade every enemy mid laner if not in health, in mana (though usually in health too). Leblanc trades off NOTHING for the very large amounts of power that she possesses. I honestly can't say I can't say I can think of a single weakness Leblanc possesses, besides short(ish) range.
I appreciate the ideas you and others have brought to the discussion, but I think these are attempts at bailing water out of a sinking ship. I have to admit that none of the ideas that have been mentioned here are particularly satisfying ideas and all lead to me to believe that she needs some changes.