Damage redirection + reduction and mass dmg

1) Damage increases and reductions are recalculated each time damage is redirected. In your example, the three damage redirected to Stealth Bot is reduced by Stealth Bot's -1, and so Stealth Bot takes 2 damage. Legacy's effects are no longer applied. 

2) In every case multiple targets need to take damage, damage happens one target at a time and players choose the order targets take damage in. Ra's Scorched Earth is a great example. Say you are in Insula Primalis and there are 2 envrionment cards in play, and say they are 2 Velociraptor Packs with 2 HP each. Ra will deal 2 damage to each non-hero target. You select the first Velociraptor Pack and it takes 2 damage, destroying it. Now the second Velociraptor Pack only takes 1 damage becasue there is only 1 envrionment card in play. If you hit the VIllain before all of this, they would take 2 damage. If you hit them after, they would take 1. 

Hope that helps!

edit: Forgot to mention. This thread is by the developer of the videogame, which catologues major rules clarifications and updates in order of expanions. It's probably one of the best places to check initially if you have a question about a certain card.  

Caveats to #1, though:

If no damage would be dealt, it is not redirected. Say Wraith has used her base power and would be dealt 2 damage. Even if there's something that would force the damage to be redirected (say. Immutus), and it would be more than 0 damage to a different target, it doesn't. Wraith takes 0 and that's the end of it.

Also, one-time damage reduction, like Wraith's, can be used up even if it is not used. Take the above example: if 3 damage is pointed at Wraith, so that she would take 1, instead Immutus takes 3 but the damage reduction was still "checked," so it's still consumed.

In addition to the above regarding redirection, don't forget to take Nemesis bonuses into account, if applicable. For example, the Chairman tries to hit Mr Fixer for two points of damage. This is increased to three because of the Nemesis bonus.. But say Fixer has his Pipe Wrench out, thereby reducing all damage to him by one. And he also has Driving Mantis, so when those three points of damage are reduced to two, they suddenly become eligible for redirection and he decides to send them to the Operative. The damage is "reset" to its original two points as we are now recalculating for another target. But like Mr Fixer, this one also shares the Chairman's Nemesis bonus so the damage goes back up to three again. Since the Operative has no means of reducing or redirecting this, that's how much she gets hit for :D.

And for your second question, I think that's been pretty well covered but note that it's not just damage that this applies to - any time any effect will hit multiple targets, players always get to choose the order - nothing is really simultaneous. So for example, say Fanatic/Apostate have just let off an End of Days/Apocalypse while there's a whole bunch of stuff out across all decks. Say Unity is present in this game and has a Bee Bot out. You can destroy all applicable villain and environment things that you want to get blown up, then destroy Bee Bot, who can interrupt the End of Days/Apocalypse by destroying it, which means it's gone and nothing else gets blown up. So other than Bee Bot, everyone gets to keep all their stuff (unless they're someone who likes it when their stuff breaks, like Mainstay or Stuntman or someone, in which case they may ask for that to get popped before the bot) :D.