If any creature cards put onto the battlefield this way become permanents that aren't creatures because of another effect, those permanents won't be Zombies, but they will gain decayed.

Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.

Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.

Decayed represents a static ability and a triggered ability. "Decayed" means "This creature can't block" and "When this creature attacks, sacrifice it at end of combat."

The creatures will enter the battlefield simultaneously. They will see each other enter for the purposes of triggered abilities.

Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.