Fatras, le protéiforme

Although the tokens enter 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.

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

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.

Except for the listed exception, Muddle copies exactly what was printed on the target creature and nothing else (unless that creature is copying something else or is a token; see below). 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.

If an instance of 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.

If the copied creature is a token, Muddle copies the original characteristics of that token as stated by the effect that created that token, with the stated exception.

If the copied creature is copying something else, then Muddle becomes a copy of whatever that creature copied, with the stated exception.

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

If the target creature is an illegal target as Muddle's ability tries to resolve, it won't resolve and none of its effects will happen. Muddle won't have myriad.

Muddle's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.

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.

The tokens created by a single instance of myriad all enter at the same time.

When Muddle becomes a copy of the target creature, it's neither entering nor leaving the battlefield. Any enters abilities or leaves-the-battlefield abilities won't trigger.

While Muddle is a copy of something else, it won't have its own triggered ability. However, if Muddle still has abilities on the stack waiting to resolve, they will do so. (It won't get multiple instances of myriad this way.)

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.