If the mana is spent on a Dragon creature spell, that creature will have hexproof as soon as it enters the battlefield. There is no opportunity to target the creature before it gains hexproof.

If more than one produced by a Jade Orb of Dragonkind is spent to cast a single Dragon creature spell, the delayed triggered ability associated with each mana spent will trigger. That creature will enter the battlefield with that many +1/+1 counters.

The mana created by Jade Orb of Dragonkind can be spent on anything, not just Dragon creature spells.

The delayed triggered ability will trigger whether Jade Orb of Dragonkind is still on the battlefield or not.