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

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 any costs to have a creature attack, those costs won't apply to the tokens.

The tokens all enter the battlefield at the same time.

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.

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 at the time it became an attacking creature this combat, or the controller of the planeswalker or protector of the battle the creature was attacking at the time it became an attacking creature this combat.

If you choose not to have Auton Soldier enter the battlefield as a copy of another creature, it won't have myriad. It will also probably die due to having 0 toughness.

Each token created by the myriad ability will be a copy of whatever Auton Soldier is copying, along with the listed exceptions. However, it doesn't copy whether Auton Soldier 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.

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

You choose whether each token is attacking the player or a planeswalker they control as the token is created.