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

Poppet Factory's first ability removes all abilities from creature tokens you control, including ones they were created with and ones they gained since the last time Poppet Factory transformed.

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

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."

As Poppet Factory transforms back into Poppet Stitcher, the effect making your creature tokens 3/3 and making them lose their abilities ends. Their base powers and toughnesses will revert to whatever they were before that effect started applying, although other effects that modify power and/or toughness, such as ones created by Auras, counters, and the like may still apply. They'll also have whatever abilities they had before they lost them.

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