That 17 str is whats killing you.
Id roll with a 16 str and put those points to dex...the little bit of damage you get is nothing next to your Sneak Attack damage, and having a better balanced toon will serve you better. You will gain +3 to dex from the enhancements for acro II which will help, but I would take 2 ranks of traps sense minimum to aid you there.
The other thing to think of is your reflex is not just traps its also avoiding all those nasty damage spells with evasion.
Drop cha down to a 10 or 8 and put those to Wis/Dex ac+reflex will serve you better
As for UMD you will be fine when you get some items...and some levels. I rolled with an 8 cha.. and have a base umd score of 41 at 19 level... you will be a-ok.
A 14 int is more than enough ... your diplo will be a-ok . Even without the top diplo score in the game I have no issues shedding aggro.
My $.02