Fire Demon or Mana Relfection?

Comp is not 100% chance (70% chance in description)
or they changed?

its hard to know if the Comp's DoT works esp in a party boss run

It'll be easier to tell once Chaos hits and you can actually see how much your DoTs are ticking for (of course this doesn't apply to party runs). But honestly at 80%, I would just use it once every 10 seconds and if it doesn't proc, oh well. It's worth it to use it, but it's not worth it to keep using it to make sure it's up.