There's 2 separate things involving auras and protection. The first says it can't be targeted. A creature with protection from white cannot be targeted by a white aura (as we all already know, of course). This is what stops Faith's Fetters from being cast on the red Akroma. Replenish gets around this though, because you're not casting the aura, you're just putting it on the battlefield so you simply choose an object to attach it to, no targeting involved. Because of this, you could use Replenish to put a Faith's Fetters on a Multani. But protection from white also means it cannot be enchanted by white auras. When putting the Faith's Fetters onto the battlefield, you must choose an object it can legally enchant to attach it to. Since red Akroma has protection from white, it's not such an object, so you have to attach it to something else. An aura being attached then falling off typically involves the object it's enchanted to gaining protection after the aura is attached, not before.
As for Animate Dead + white Akroma, let's first take a look at the oracle text on Animate Dead:
Animate Dead {1B} |Enchantment -- Aura| Enchant creature card in a graveyard / When Animate Dead enters the battlefield, if it's on the battlefield, it loses "enchant creature card in a graveyard" and gains "enchant creature put onto the battlefield with Animate Dead." Return enchanted creature card to the battlefield under your control and attach Animate Dead to it. When Animate Dead leaves the battlefield, that creature's controller sacrifices it. / Enchanted creature gets -1/-0.
When you cast Animate Dead, it has "Enchant creature card in a graveyard." So you have to target the Akroma in the graveyard at this time. This is perfectly legal because the protection ability of Akroma only applies while it's on the battlefield. Then the enters the battlefield trigger of Animate Dead triggers and when that resolves, a whole mess of things happen. First it loses its enchant ability, then it gains a new enchant ability. While these things happen, it's still attached to the card in the graveyard. It doesn't fall off yet, even though after each step it's enchanting an illegal object, because that happens as a state-based action, which doesn't get checked mid-resolution. The next step is to return the creature card to the battlefield. After that, you attach the animate dead to it.
Here's where things break down. Right before this step, we have Akroma on the battlefield and Animate Dead with "enchant creature put onto the battlefield with Animate Dead" now not attached to anything (the object it was attached to in the GY disappeared). We're instructed to attach the Animate Dead to the Akroma. From Animate Dead's point of view, this is no problem. The Akroma is a legal object according to Animate Dead's enchant ability. But from Akroma's point of view it is a problem. Being on the battlefield its protection from black now applies, so it cannot have a black aura attached to it. So the "attach Animate Dead to it" fails to happen. The final step is creating a triggered ability that will trigger when Animate Dead leaves the battlefield.
Now that the ability is done resolving we have a State-Based Action check. At this time we have an aura, Animate Dead, that's not attached to anything. So it gets put in the graveyard. This causes Animate Dead's newly granted "leaves the battlefield" trigger to trigger. It's important to note that this triggered ability refers to "that creature" (the one returned from GY to battlefield by the EtB trigger) and not "enchanted creature." There is no enchanted creature, but the Akroma is the creature that was returned by its ability, so it is "that creature." So when this triggered ability resolves, Akroma gets sacrificed.
To sum it up about as short as possible: Akroma enters the battlefield, Animate Dead never gets attached to it, then Animate Dead goes to the graveyard, then Akroma gets sacrificed.
Necromancy is just a little bit different because it's not an aura when you cast it, so doesn't target anything until it's on the battlefield. Other than that small difference, all 3 cards work the same in this interaction.
As for how it used to work, I typically prefer taking the 1984 approach and once the change is made just working from an "it always worked this way" standpoint because discussing old wordings and no longer working interactions can lead to people who are brand new to the subject getting confused. But, as I'm sure many of you are questioning if how you thought it previously worked is correct and what's different about it now, I'll make an exception here. It used to be that you'd get to keep the Akroma and the Animate Dead would be in the graveyard. There were a number of differences in the oracle wordings (I'm not going to dig up the old wording, that can be an 'exercise for the student') including Animate Dead and Dance of the Dead not being auras when you cast them, using a clause similar to what Necromancy has now. The big relevant difference though was the trigger that told you to sacrifice the creature when Animate Dead left the battlefield referred to "enchanted creature." Since Animate Dead was never attached to the Akroma, there was no enchated creature to sacrifice. This changed happened about 3 or 4 years ago.
