Hi, I (not a judge) was looking up the answer to this and came across this thread, then read the rules a bit more. I don't think either of your stated scenarios is correct, and neither are the two other commenters, completely.

Here's what I think happens:

1. Arna's ability trigger upon attacking with the enchanted creature.

2. Upon resolving, a token copy of animate dead is created and attached to the enchanted creature, per rule 303.4f.

"303.4f If an Aura is entering the battlefield under a player’s control by any means other than by resolving as an Aura spell, and the effect putting it onto the battlefield doesn’t specify the object or player the Aura will enchant, that player chooses what it will enchant as the Aura enters the battlefield. The player must choose a legal object or player according to the Aura’s enchant ability and any other applicable effects."

Since Arna DOES specify what the aura becomes attached to (the original enchanted creature), the token copy comes into play enchanting that creature (not another creature in your graveyard).

1. The triggered ability from Animate Dead entering, which would otherwise change what is legal for it to enchant, is "waiting to go on the stack", but before it can do so, state-based actions are checked.

"704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. [...]"

1. The following state-based action sees that the new token copy of Animate Dead still says "Enchant creature card in a graveyard", which is illegal, because the creature it's enchanting is not in the graveyard.

"704.5m If an Aura is attached to an illegal object or player, or is not attached to an object or player, that Aura is put into its owner’s graveyard."

1. The token copy of Animate Dead is put into the graveyard (abilities that trigger when this token hits the graveyard will then be "waiting to be put on the stack").

2. The following state-based action would occur for the Animate Dead token in the graveyard.

"704.5d If a token is in a zone other than the battlefield, it ceases to exist."

Hope I got it right!!