For explicit clarification, for the purposes of effects such as Spite's DR, Fixer's Crane Stance, and Nightmist's redirection, does each hero's turn reset these effects, so that Mr. Fixer could redirect the damage from a Rampage, a Grenade Launcher and a sufficiently weak Oblivion during a single round, or is he limited to once per 'overall hero turn?'
Similarly, and especially for Nightmist, does 'the first time' specifically only refer to the first time Nightmist would take damage, or can she choose to ignore redirecting 1 damage to redirect 4 damage that would hit her instead, such as if she was infected and decided to heedless lash Plague Rat?
Each individual game turn resets these effects. Each hero gets a turn, the environment gets a turn, and the villain gets a turn, and Fixer/Nightmist could redirect on every single one of them.
You can't ignore the smaller damage to wait for the bigger damage. "First time" means first time, similarly to how Wraith's damage reduction innate blows everything you've stacked up the next time you take damage, even if it's only a plink.
The "first time" means the first time the condition is met, if you can chose not to use the redirect then you can chose not to use the redirect but you can't choose to redirect something else later as it is no longer "the first time" you are damaged this turn.
but does "first" trigger on every instance of damage?
so if I hit with an attack with multiple instances (eg.: damage written on multiple lines, or multiple types of damage), the "first each turn" condition only triggers on the first instance, or is it on the card as a whole?
The trigger is on the first attack. If a card makes two attacks against the villain and the villain triggers based on the first attack each turn, then the trigger only goes off once.
I think the question is, for example, would Fanatic's power which does two types of damage, considered one attack or two. I think the answer is that it's a single attack because it's all packaged together.
I would disagree. Most of these effects don't say "the first time you're attacked", they say the first time you're damaged. Fanatic's attack is 2 separate instances of damage (both damage boosted individually and damage resisted indvidually). It may be a single attack, but it's 2 instances of damage, and that's (to my knowledge) all that cards refer to.