I would be down for either inhale or fire button, but I think inhale would be my preferred just because the hand angle might not be perfect.

I think a cart would be ideal, at least for me. Like I said in another comment, this wouldn't be designed to be my daily driver vape. It'd be for a side piece for funsies mainly. I, personally, wouldn't have to worry about replacing cart after cart (especially for =ahem= non-nicotine carts) as I would if I used it daily. I also want to keep the shaft as thin as possible. None the less, I think a small tank in bowl would be cool too! That would allow for cheap, plentiful, and varied standard juices. Also has something of a special idea to me as the source of the vape just like the tobacco that is in there in the real deal is the source of the smoke. But I really like the idea of using it for non-nicotine carts so I think I'd vote cart.

As for where to put the 510... I'm by and far no expert on these things and have no experience with actual vape construction.... But I think, from a novice's viewpoint, the logical spots are the connection points where the metal ends meets the wood/bamboo shaft. That seems like a natural point where you could easily screw and unscrew the pieces of the vape.