How about Meticulous Concoction?
Firebug and Reactionary are both combat traits, you can only have one trait from any trait section such as combat traits.
When selecting traits, you may not select more than one from the same list of traits (the four basic traits each count as a separate list for this purpose).
Pragmatic Activator and Focused Burn are both magic traits, again, you can only select one of them.
Alchemists have weak will saves, a +1 Will save bonus from something like Indomitable Faith can help offset this.
Firebug (the alchemist version, not the kobold version) and Reactionary are both combat traits. They can't coexist.
I like Pragmatic Activator a lot because you can just dump Charisma to oblivion and still be extremely good at the things you should be good at. However, the Alchemist has a very good spell list, so you might not even need to UMD, at least not for Wands. Scrolls are a different story, though, so UMD is still valuable. And being able to fool magic items is rarely useful, but when it is, it's extremely so.
As an aside, why are you using a Crossbow? As a Grenadier, you get a free martial weapon proficiency, so why not use a Longbow? Higher range and fast reload by default which gets really great when you can regularly apply alchemical weapons or bombs to your projectiles.
Crexis, generally when people reference a weapon they are using it is the weapon they plan on sticking with, not a temporary weapon.
If it is a temporary weapon I suggest letting us know that.