Status of non-top mutated creature cards

Question

Ikoria's new Mutate ability allows two or more creature cards to be fused into a single creature, with the creature on top gaining the abilities of the creature cards below it.

From a rules perspective, what exactly are the non-top cards? While they wouldn't be considered creatures in their own right, are they still considered permanents? Are they still targetable? Do they count as being on the battlefield?

I'm primarily curious about two specific edge cases:

1. Is it valid to play a "Destroy target nonland permanent" targeting a non-top mutate creature card? If so, would the entire mutated creature be destroyed or would the targeted creature card be effectively removed from the mutation?
2. If a Legendary creature card is a non-top card in a mutation, is a Legendary with that name still considered to be on the battlefield for the purposes of casting a second Legendary with the same name? If it still counts as being on the battlefield, what happens when the second Legendary is played and you choose to get rid of the first one? (I suppose that partially boils down to the above question as well.)

As the official rules haven't yet been released (to my knowledge), I'm aware that some of these questions may be difficult to answer. However, there may already exist related or similar rules/mechanics (that I'm not aware of) that already provide a clear answer to such situations. Any relevant quotes from Wizards employees may also shed some light on the subject.

Answer 1

The cards are considered only 1 creature; 1 permanent. The cards on the bottom do not exist in any meaningful sense as their own thing. They are not objects in the game; so nothing could target them. A bottom card has no types, no name, no mana cost, nothing; so it would not interact in any way with the legend rule.

From the Ikoria mechanics article:

As a mutating creature spell resolves, instead of entering the battlefield, it merges with its target into one creature.

Emphasis mine. The new spell with mutate, if cast for its mutate cost, never actually enters the battlefield.

You'll end up with a pile of cards (or tokens—we'll get to that in a second) that all represents a single creature. That creature will have all the characteristics of the top card, and it will also have the abilities of every card underneath.

So there is only a single creature. The only text on the non-top card that has any meaning is the abilities that it grants to this single creature.

Answer 2

The Ikoria Mechanics Article has the following paragraph about using the Mutate ability:

As a mutating creature spell resolves, instead of entering the battlefield, it merges with its target into one creature. Its controller chooses to put the resolving mutating creature spell on top of its target or underneath it. You'll end up with a pile of cards (or tokens—we'll get to that in a second) that all represents a single creature. That creature will have all the characteristics of the top card, and it will also have the abilities of every card underneath.

The whole pile represents a single creature, which seems to imply that it is a single object from a game perspective, which would mean that the cards in the pile can only be interacted with as a group, not individually.

It says that the creature has the characteristics of the top card and the abilities of the other cards, which implies that it does not get any other characteristics from any but the top card.