40k is a somewhat intolerant setting for other races, elfdar stick out too easily if their own arrogance didn't give them away, ogryn are too dumb for an investigation game. Ratlings MIGHT work, but what do they bring? The other abhumans all stick out in their own way, Kroot won't work, I could never see an Ork working either. RTs can break the rules a bit because they spend most of their time out of the imperium, while acolytes are the exact opposite. MAYBE in a future book they can have an addon like Grey Knights did for Malleus. But for the most important book? The Inquisitor's Handbook of DH2? No thanks. Look at how much skimpier Into the Storm is compared to the Inquisitor's Handbook, it tried to do too much and ended up spread too shin, while the Handbook is thick and packed full.
Eldar are the ones who really could blend into imperial society more easily if they really want to.
Their body shape and even faces are similar to humans. On most planets they could easily be considered as some off-worlder-humans, if they use a little make-up and corretly chosen clothes.
The Imperium is so big, that the "classical human" does not exist. Just think about comparing a bulky feral worlder to a thin pale void born.
I think ther eis even a novel where blending in of an eldar happens without too many problems.
Of course, other inquisitors or people with xenos knowledge would know. But such are rare and can be avoided.