The first thing I have to say would be directed to your DM that he should definately let you guys tweak and change your existing characters to deal with the loss of the other player. Its in the best interests of everyone - 3.x is a system that rewards character planning and punishes haphazard choices. You guys shouldn't be punished because another player left.
The second thing would be to the player of the Artificer that they should step and rejigger their character to take on the trap finding-disarming role. They have Trapfinding and Search, Disable Device and Open Locks as class skills so they can fill that roll without even having to multiclass. Removing that responsibility from you lets your rogue skill points cover your social abilities as you multiclass to beef up your combat resilience. Also, if you were putting points into being a scout (Hide and Move Silently), that would be a role that the Ranger coud fill.
You know, as odd as it sounds, I think that 16 is getting wasted in Charisma. My view is that the most important thing for a social character is that they have enough skill points to keep the relevant skills maxed. Those skillpoints (don't forget the synergy bonuses!) are going to do the heavy lifting. Now that you're taking on more combat, I'd think that you'd be most optimized with a 12 in Cha and Con bumped up to 16.
From a game mechanics standpoint, Two-Weapon Defense became worthless when they added Improved Buckler Defense. Personally, though, I think Bucklers should work like the Dodge feat (applies to one target only) and even then I'd be iffy on allowing that feat in a campaign.
You should definately consider what three levels of Swashbuckler give you. You get +3 BAB, 3d10 hit die, Weapon Finesse and can add your Int to damage. If your int is +4 or better (not too hard with buff magic) that's effectively the same as 1d6 of sneak attack damage. And here's the kicker, 4 skill points per level and Bluff, Diplomacy, Sense Motive and Tumble are all in your class skill list.
Further down the line, the Tempest PRC could help as well. Over five levels you're getting 5d10 hit die,
+7 BAB, and +3 to AC. And while spring attacking means you're not getting in all of your multiple attacks, it is a good way to help you stay alive and the two weapon spring attack effectively lets you spring attack as well as a straight rogue. You've got less sneak attack damage per attack, but you're hitting twice.
My plan for a TWF-sneak attack damage that can survive ticking off the enemy looked like this:
Code:
lvl Class BAB feats & (class abilities)
1 Rog 1 0 TWF, (sneak attack 1d6)
2 Swa 1 1 (Weapon Finesse)
3 Ftr 1 2 Dodge, Mobility
4 Rog 2 3 (Evasion)
5 Swa 2 4 (Grace +1)
6 Swa 3 5 Spring Attack, (Insightful Strike)
7 Ftr 2 6 Improved TWF
8 Rog 3 7 (sneak attack 2d6)
9 Tmp 1 8 _______, (defense +1)
10 Rog 4 9
11 Tmp 2 10 (Ambidexterity -3/-1)
12 Tmp 3 11 Greater TWF, (Defense +2, TW versatility)
13 Rog 5 11 (sneak attack 3d6)
14 Tmp 4 12 (Ambidexterity -2/+0)
15 Tmp 5 13 _______, (defense +3, TW spring attack)
Finish up with 5 levels of Rogue and you've got something that has a BAB of 19, averages d8 hit die, and adds Int + 5d6 to sneak attacks.
Your being a changeling with favored class of Rogue complicates things a bit as that class order would give an XP penalty during level 6. You can tweak the order, but it basically means that you'd have to wait until level 9 to get Improved TWF.
On the other hand, here's a case for why its may be okay to just grab as many rogue levels as fast as possible:
* Crippling Strike as your 10th level Rogue ability - inflicts 2 points of Str damage every time you hit with a sneak attack, this was *made* for a TWF'ing rogue
* Staggering Strike (feat from Complete Adventurer) - every time you do a sneak attack, foe must make Fort save equal to the damage done or be staggered (can only take a standard action) for one round. Limits the damage that can be done back at you.
Warshaper is pretty uber - you made a good call there. No crits and +4 to str and and con is huge. The third level is also powerful, but would be best utilized with some kind of reach polearm using build, so that you could make full attacks from a range not reachable by a 5' step. The spiked chain is a finessable reach weapon, but it doesn't fit in with your TWF concept.
That was a pretty rambling post, but hopefully there's some stuff in there that will be useful to you in figuring out what to do.