2014-11-07 : The sacrifice isn’t optional. If you control at least one creature, you must sacrifice one. If you control no creatures, you won’t sacrifice one and you won’t draw two cards. The same is true for the chosen opponent.

2014-11-07 : You choose the opponents for each effect as the spell resolves.

2014-11-07 : You choose which creature card you’re returning to the battlefield, then the chosen opponent chooses which creature card they are returning. These choices aren’t made until Infernal Offering resolves. Neither card is a target of Infernal Offering.

2014-11-07 : You may choose the same opponent for each of the effects, or you may choose different opponents. None of the affected players are targets of the spell.