How does scaling work with infernalist fire damage?

So about the whole fire damage calculation. So far I've read that damage and magnitude both contribute to the ignition damage, and firewall does ignition by default. Is getting magnitude better or fire damage better? My understanding is that +10% magnitude adds only about 2% (22%) more ignition dot while +12% fire damage could make the end damage slightly more is that the case?

Also does fire resistance pen help with ignition damage and by how much?

And last question, how does "gain 8% of damage as extra fire damage" work with all these conditions? Does it just add 8% to the hit or to everything kinda like multiplicative?

Also, area of effect doesn't seem to affect firewall effective range at all. So what does that do for the spell?

Thanks for any answer or discussion!