#changing zones

[magicjudge]

If I have Elenda on the Battlefield and get hit with a board wipe, then in response cast something like Supernatural Stamina. How does that resolve? Will Elenda get the counters from things dieing to the board wipe since she returns to the Battlefield? How many tokens will she make? Is it equal to her power when the board wipe went off or her new power after coming back from the graveyard?

Unfortunately for you, Elenda is a new object after changing zones and so all the triggers from creatures dying were created by an Elenda that no longer exists and thus won't end up putting any counters on the new Elenda.

As for how many tokens you make, it's based on Elenda's power when she died, but that will include the +2/+0 buff from Supernatural Stamina.

[magicjudge]

I have Oskar Rubbish Reclaimer on board. I discard Counterspell to cast it. Then I discard Shadow of the Grave and cast it. What cards are returned to my hand?

When you cast Counterspell from your graveyard after discarding it, it becomes a new object leaves the graveyard (and again when it returns there). This means that it's not a card you discarded this turn, according to Shadow of the Grave and won't go back to your hand.

[magicjudge]

Scrappy Bruiser has the text "Whenever Scrappy Bruiser attacks, up to one target attacking creature gets +2/+0 and gains trample until end of turn. Return that creature to its owner's hand at end of combat. (Return it only if it's on the battlefield.)" The part in parentheses is reminder text, meaning it's a consequence of the other rules on the card rather than a functional part of the card. Why is it not necessary to specify that as part of the rules text of the card?

Whenever a card changes zones, e.g. battlefield to graveyard, it becomes a new game object with no connection to its existence in the previous zone.

This means that effects like Scrappy Brawler's trigger can't track the creature to its new zone unless they specifically mention that zone. Since Scrappy Brawler's ability doesn't mention another zone, its delayed trigger will only be able to do anything if the creature hasn't changed zones in the interim.