Delve doesn't change a spell's mana cost or mana value. For example, Treasure Cruise's mana value is 8 even if you exiled three cards to cast it.

You can exile cards to pay only for generic mana, and you can't exile more cards than the generic mana requirement of a spell with delve. For example, you can't exile more than seven cards from your graveyard to cast Treasure Cruise unless an effect has increased its cost.

Because delve isn't an alternative cost, it can be used in conjunction with alternative costs, such as flashback. It can also be used to pay for additional costs that include generic mana.

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.