You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.

You may target any creature, not just one that dealt damage to you this turn.

Inferno Trap's alternative cost condition checks for any damage, not just combat damage.

The alternative cost condition is satisfied only if two different creatures dealt damage to you, not if one creature dealt damage to you twice. The damage may have been dealt to you simultaneously (as with combat damage), or at separate times.

The alternative cost condition doesn't check who controlled the creatures that dealt damage to you. Damage dealt to you by your own creatures counts.

Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.

Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.