Will of the Naga can target creatures that are already tapped. Those creatures won’t untap during their controller’s next untap step.

Will of the Naga tracks the creatures, but not their controllers. If any of those creatures changes controllers before its first controller’s next untap step, then it won’t untap during its new controller’s next untap step.

If you chose two targets and one is an illegal target as Will of the Naga resolves, that creature won’t become tapped and it won’t be stopped from untapping during its controller’s next untap step. It won’t be affected by Will of the Naga in any way.

You exile cards from your graveyard at the same time you pay the spell’s cost. Exiling a card this way is simply another way to pay that cost.

Delve doesn’t change a spell’s mana cost or converted mana cost. For example, the converted mana cost of Tasigur’s Cruelty (with mana cost ) is 6 even if you exile three cards to cast it.

You can’t exile cards to pay for the colored mana requirements of a spell with delve.

You can’t exile more cards than the generic mana requirement of a spell with delve. For example, you can’t exile more than five cards from your graveyard to cast Tasigur’s Cruelty.

Because delve isn’t an alternative cost, it can be used in conjunction with alternative costs.