How does ignite / how do ignite supports work?

I'm feeling slightly like I've been coaxed into a snafu here as Fire Sorc - damage so far has been miniscule and I'm not really seeing any ways to scale it.

That aside, how does ignite actually work? Firewall and incinerate definitely feel like they stack. Do two firewalls stack? Do multiple ignites from incinerate stack?

Does 'chance to ignite' from e.g. Ignition actually affect either of these skills? Do they have a fixed 100% chance to ignite per tick, or just a base 100% chance that can be raised and lowered?

Eternal Flame is a suggested support for incinerate, but does increasing the ignite duration do anything meaningful? Isn't it about to be refreshed by the next tick of incinerate anyway, or are there actually discrete stacks being applied? If the later, does cast speed do anything to incinerate besides boost stage growth?

Eternal flame supports skills that Hit. Do Incinerate and Firewall 'hit', even though they do no damage besides applying ignite? Does Fortress reduce Firewall damage when it causes "less damage with Hits"?

I'm just realising penetration doesn't affect ignite at all, so that's a few points saved... Does "damage with hits against burning enemies" increase the magnitude of ignites applied by incinerate and firewall?