I posted these questions here and sent them to GW. A GW representative sent me this back in an Email. 1. If a skink chief is mounted on a stegadon, can he fire the weapons attached to it (one blow pipe or giant bow)? He does replace a crew member, who can use them. He can fire one blow pipe or help fire the giant bow. 2. If the answer to #1 is yes, can he use the "Venom of the Firefly Frog" enchanted item to get rerolls to hit on the already poisoned attacks from a mundane weapon? No. Venom of the Firefly Frog only affects his personal weapons, not the weapons on the Stegadon.
Sounds very similar to this thread. Really, GW staff can't give answers that get taken as official, they can just give their interpretation like the rest of us. Only the designers of the book can say how the rules were intended to be used. As I mentioned elsewhere, the venom says it can be used on anything mundane the chief can use so by the letter of the rules yes you can. From a balance perspective, it is a bit powerful and I wouldn't even try to use it. Good to see that lots of people are saying nothing wrong with chief's firing the bow though. The added BS makes it a lot more usable.