I dont really think the alchemy and poison use are good fits for how i imagine a investigator that being said the inspiration rules are golden and considering it will recharge in the same fashion as spell points why not just have the class draw more heavily off of that? even possibly allowing them to use inspiration points to cast certain spells of certain levels as sla's much like how the qinggong monk can use ki points to cast certain spells.
with more inspiration point options,a healthy list of investigator exclusive talents (not discoveries) and a few more options it would be a awesome class that feels different and fits the class name.
Also instead of sneak attack what if there was a class feature or talent that allowed the investigator to spend inspiration as a swift action to gain the sneak attack equivalent of a rogue of its level for a round?
And what if poison use and swift poison became investigator talents (swift poison requiring poison use).