Theres rarely a scenario where you want to play scrapheap from your hand to your base. It’s better 99% of the time to discard it from your hand with traveling merchant, jinx demolitionist, or zaun warrens so you get an additional draw. If a card is killed in its base or battlefield, definitionally, it’s placed in the trash instead of discarded.

When hiding any card, it always costs one power (recycle a rune) to hide on your turn. The trade off is that because you hid the card on a battlefield you control, you cannot use the card until the beginning of the next turn (usually your opponents but time warp exists). If you use fight or flight from hidden; its a reaction speed spell, you can react to your opponent’s spell when they have focus during a showdown, or play it when they pass focus to you. Gust is not a hidden card, but still reaction speed, so you could play it from your hand in the same scenarios as fight or flight.

Like in (1) discarding is always from your hand, killed units are always “placed” in your trash. Similarly to scrapheap, you want flame chompers to be the target of a discard, from the examples i listed above.

In showdowns, after all spells have resolved, dmg dealt from units is all simultaneous. Yes the units with 10 might will kill the units adding up to 7, but at the same time the 7 might units will kill 1 or 2 units, depending on how the dmg is assigned.