Super Polymerization

Regarding "Neither player can activate cards or effects in response to this card's activation":

It is not treated as an effect.

Regarding the effect of ①:

It cannot be activated during the Damage Step.

When resolving, the process of "Fusion Summon using monsters from your field and/or your opponent's field as Fusion Materials" is performed. Afterwards, the process of "Fusion Summon 1 Fusion Monster" is performed.

The process of using monsters as material and the process of Special Summoning are not performed simultaneously.

Fusion Material Monsters are sent to the GY unless another effect is applied, but Fusion Materials may be sent to a place other than the GY due to the application of other effects. In that case, if the Fusion Material Monsters have left the field, the Fusion Monster is Special Summoned.

When resolving, you can include your face-down Defense Position monsters as Fusion Materials, but you cannot include your opponent's face-down Defense Position monsters as Fusion Materials.