The tokens all enter the battlefield at the same time.

You choose whether each token is attacking the player or a planeswalker they control as the token is created. If it's attacking a planeswalker, you choose which one. You can't have any of the tokens attack a battle.

Any enters-the-battlefield abilities of the copied creature will trigger when the tokens enter the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the copied creature will also work.

If myriad creates more than one token for any given player (due to an effect such as the one Doubling Season creates), you may choose separately for each token whether it's attacking the player or a planeswalker they control.

Each token copies exactly what was printed on the original creature and nothing else. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

The term "defending player" in the myriad rules (or any other ability of an attacking creature) refers to the player the creature with myriad was attacking, the controller of the planeswalker it was attacking, or the protector of the battle it was attacking at the time the ability resolves. If that creature is no longer attacking, it refers to the appropriate player based on who or what the creature was last attacking.

Although the tokens enter the battlefield attacking, they were never declared as attackers. Abilities that trigger whenever a creature attacks won't trigger, including the myriad ability of the tokens. If there are any costs to have a creature attack, those costs won't apply to the tokens.

If the defending player is your only opponent, no tokens are put onto the battlefield.