Here's the section relevant to your question however;
There's an entire section on Elemental Damage as well. I didn't find any specific reference to Magic Weapon: Ignite, but Elemental Damage almost always applies to any damage you do, so I would say it's a safe bet to say it will increase the damage of Ignite. If anyone knows for sure, they are welcome to prove me wrong howeverMagic Weapon - Ignite
DoT stacks. DoT can crit (one crit roll for all ticks). DoT ticks have a proc coefficient of 0.
DoT can be procced by anything that has a proc coefficient. The chance for each of those skills to apply the DoT is equal to their respective proc coefficient.
Course, you could always test it yourself, using one set of gear without Elemental Damage and another set with it (both with similar damage ranges obviously) and see if the Fire set ticks higher or not.