Chaos, I seem to remember reading somewhere that all of the inquisitor's spikes (with the exception of their lynch pin and their eye spikes) are in their chest. However, I'm having trouble remembering where exactly it was that I read that, so I could be (and probably am) totally wrong. However, it would not be a good idea to pierce the stomach enough that the acid could drain away. Stomach acid is potent stuff. If I remember correctly, it's a type of hydrochloric acid, which puts its pH at 1. Having that stuff swishing around loose in your body...not good. Although I guess that could be (yet) another explanation for why inquisitors need to rest often.
Anyway, back on track....
I personally believe that the inquisitors do not ingest atium every morning, as it seems impractical. However, that requires that they have some way of getting it fast and before they should technically know they are going to need it, or that they have some other ability (*cough*hemalurgy*cough*) that produces the same effect as atium.
As a side note, atium would probably be completely useless to inquisitors. The only thing that might happen is that they see where the metal lines would be in a few moments, which would be REALLY confusing. So, wouldn't that mean that the only reason an inquisitor burns atium is to confuse the mistborn he's fighting?
My head hurts when I think about only being able to see via metal lines. When I factor in being able to see the equivalent of 20 or so extra people...ouch.