Point#1: possibly insoluble. Good better best, small medium large, tall venti grande. There was a better play available and it didn't get chosen.
You can't reduce a villain's HP to 0 (which is the primary goal in the vast majority of cases in the game) without dealing that villain damage.
That's true. I did not say "never do damage," though, just that it's often useful to do less now in order to improve the board state.
It is possible for a hero to have more than one role.
I agree, but roles are just labels we stick on them to describe in simple narrative terms the aggregate impacts of their cards. You said "Since Tempest is meant to be a damage dealer first, having a card that deals a lot of damage without a drawback is a huge positive" and to me that seems like a tautology that shouldn't impact how we evaluate Tempest's cards. Having a card that deals a lot of damage is a huge positive, his role doesn't change the evaluation at all - there is no "since."
Also, in the context of the reanimator combo team I posted above, I guarantee you that Reclaim is Tempest's best card, and his main role is not direct damage, it's combo enabler.
"Into the Stratosphere" is a nice card, but it's not a long-term solution for anything
Neither is Take Down unless it's perma-looped (and even then, it doesn't actually win the game for you by itself). But you gave Take Down 5/5 because "being able to slow a villain down helps give the player that extra push toward victory. In some cases, it buys the player time if they’re taking longer to get the proper setups and combos going." Take Down results in Legacy taking damage while Into the Stratosphere results in a villain taking damage. Take Down *does* work significantly better against enemies that can play more than one card per turn, but they both impact tempo similarly against opponents who play one card at a time. Take Down says to the villain "board state stays the same, your next card play is two turns from now" while Into the Stratosphere says "board state is -1 thing, that thing will be played again next turn, your next novel card play is two turns from now." Given that most villains have wild power fluctuations between cards in their decks (Citizen Spring vs Devastating Aurora), delaying one of these big cards even by a turn can make a huge difference (for example, if you got to keep Otherworldly Resilience, Fortitude, and Inspiring Presence in play just 1 turn longer).
Point#2: Not if you've delegated that task to someone else, no, he should do something else unless he doesn't have better cards available (which is most of them, if you've got a dedicated tank in play).
Point#3: Well, OG Baron Blade smashes the moon into the earth when enough stuff hits his trash, which is pretty strong, and while he's not that difficult his challenge and advanced modes can put the clock on you a bit harder. Apostate's Relic Spirits and Runes of Malediction like being able to find Relics and Demons in the trash. Chairman's bosses like thugs in the trash. Gloomweaver & his Chosen Disciples play zombies out of the trash, Vast Following plays Cultists or Relics from the trash. Ocular Swarms play top-decked manifestations *when* manifestations are destroyed, which is not caring about the trash but is caring about destroying things. Forced Deployment plays from the trash, and with several DR minions and/or Advanced Voss *is* a big deal for many teams.
Other situations: Gloomweaver's relics, Kaargra's gladiators, Apostate's Orb of Delirium, and several Citizens (Anvil, Truth) have hp pools that are difficult to deplete (due to size, DR, or a mix of the two). Some of these also protect other villain cards while they are out, and it can be much easier to remove them from the field, clear other things, and then let them return to an emptier board. (Edit: This, BTW, along with his excellent ongoing and environment removal, is another good reason to give Tempest 1st or 2nd position in your lineup - if he goes last, the villain card will get played again before any other hero has a chance to go, while if he goes first then his power phase and the other heroes turns will happen without the bounced card in effect).
Plague Rat smacks everyone when Infections are destroyed, not when they are bounced.
Late edits: formatting, clarification, Gloomweaver zombies.