How does that happen? Is that a break in the system mechanics or is it a feature

Trouble is, in this scenario, the dragon's the one who is a level 10 powerhouse in this scenario, while the party is likely to be level 7 when they find her.The party isn't quite to the point where their specs are truly coming together and getting nutso. Some of them will be considerably more powerful at level 7, though. For instance, the Barbarian NPC I play alongside them will have a 23 AC while raging, which actually has a chance of avoiding Nox the dragon's .50 cal sniper rifle of a bite attack, and the Rogue is going to have a 5th-level Cleric as his henchman. So, I can only hope for the best.

The NPC Barbarian is more or less filling a tanking support role. He takes hits and hinders enemies by grappling them, rather than do lots of damage. But, yeah, they've got some tactical players among them. I just hope it's enough.

Is the lady of pain on her back?

No, although the Lady of Pain will probably get involved when the Big Bads try to use a giant undead superweapon to project a negative energy blast through all of Sigil's doors and kill the multiverse. Probably.

That being everytime I tried with people who lack commitment or time.

if anyone goes into a dragon fight without matching most of it's senses, activating resistance to it's primary element, making sure you can move the same way it can or at least counter it (flight, swim, breath underwater ect), all the combat buffs in the world won't really mitigate you having a bad time

True enough. I'm kind of cluing them in that this assassin guild that one of their hated business rivals has contracted before might be involved with a dragon, but it won't be obvious. Hopefully they'll get the message, because I'm pretty sure the only ones among them who can prepare acid resistance spells are the Druid and the Rogue's Cleric cohort.

Luckily for them, though, the dragon will be so pissed they've attacked her power base that she'll fight to the death to defend her lair. It's a sewer, so she doesn't have room to fly, but she can swim through the sewage she sleeps in. She can fire her breath weapon while underwater, so I think that'll be her first move as a surprise round, followed by casting Darkness on the pool she sleeps in. Anybody gets close to the Darkness, she'll swim up and bite them with a Vital Strike Power Attack for the attack and damage I mentioned above while waiting for her breath to recharge. I'm adapting a 4E Dungeon Delve to Pathfinder, and that module suggests she will pursue and try to kill anybody who flees so she can keep her lair secret, which might be their best bet to get her into the open. I'm also planning to have the grappler Barbarian NPC I play alongside them ready an action to try and grapple the dragon when she bites at him, in an attempt to pull her out of the water. It's a longshot, but with Bless from the Cleric cohort and Inspire Courage from the Bard, it's certainly possible he'll grab her.