Dragon Change Shape feature vs Draconic Polymorph

I was curious about how the differences for a True Dragon using it’s natural Change Shape feature vs the Draconic Polymorph spell.

The Change Shape feature (text from Roll20 Compendium, Adult Gold Dragon) says this:

Change Shape: The Dragon magically polymorphs into a Humanoid or beast that has a Challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any Equipment it is wearing or carrying is absorbed or borne by the new form (the dragon’s choice). In a new form, the Dragon retains its Alignment, Hit Points, Hit Dice, ability to speak, Proficiencies, Legendary Resistance, Lair Actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its Statistics and capabilities are otherwise replaced by those of the new form, except any Class Features or legendary Actions of that form.

The Draconic Polymorph spell description (text from D&D Tools) says this:

As polymorph, except as follows: The assumed form can have no more Hit Dice than you have, to a maximum of 20 Hit Dice. Your Strength and Constitution in the new form are above the average for the race or species into which you transform; your Strength is the average for the creature +8, and your Constitution is the average for the creature +2.

So, I have a couple questions about this:

Damage: Does a dragon using the Change Shape feature only revert to it’s true form if it takes enough damage to kill said true form, or might it revert to true form if it takes enough damage to kill it’s assumed form?

Time Limit: Draconic Polymorph calls out ’10 minutes/level’ as the limit for maintaining an assumed form. Is the natural Change Shape unlimited in time?