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.
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.