Try it and see

All of the 10 channeling mages are potentially competitive. Necro and Priestess are probably 2 and 3 in popularity among that group.

I don't see what's changed. Whether you reveal within the step, after the roll, or you reveal after the step, either way you are revealing after the roll. Meaning both ways we would have the same problem and I'd be making the same arguments. Can it affect a roll that has already occurred? If it was yes, it is still yes. If it was no, it is still no. You say it was yes, but I can find no support for that answer. I would be perfectly fine with that answer if we had an official source on it, but we don't seem to and saying no to it seems easier and cleaner to me.

Ok, I'm back to my home, and a full sized keyboard:
Here's every instance of the word event in the v3-3 rules.

The player with the initiative acts first during the Action Stage, and goes first whenever you must determine the order of events.

This tells us that we may need to determine the order of events. In this context, it seems that events are just whatever things need to be put into order.

You always choose the order in which events that affect your creatures and objects occur during this phase. In the rare case that a timing issue occurs, the player with the initiative decides the order.

Example: You control a Highland Unicorn with the Regenerate 2 trait that has a Burn marker on it. Since the creature has a lot of damage on it, the Burn effect could destroy it. You can choose to resolve the Regenerate trait first or the Burn condition first.

This tells us that Regenerate and Burn are events, and by extension pretty much everything else that normally happens in the Upkeep Phase that you'd want to order.

Revealing Enchantments
Important: Hidden Enchantments have no effect as long as they are hidden! You may choose to reveal an enchantment immediately after any action or event, even if it is your opponent’s turn! This is a “free action” that does not require you to activate a creature or flip an action marker (See sidebar “When Can You Reveal?”).

This is the source of the event language that was the grease that allowed many cards to function in ways they perhaps cannot under a strict reading of the 4th printing rules.

Enchantments cannot affect an event that occurred before it was revealed. For example, you cannot reveal a Rhino Hide enchantment after the enchanted creature takes damage from an attack, to reduce the amount of damage it received.

Somewhat ironically, this is the "no going back in time" rule. It only applies to events. What's an event? Well, taking damage, in this example. The 4th printing rule is very similar.

When Can You Reveal?
You can reveal an enchantment immediately after any action or event in the game:
• At the end of any Phase of the game round. Example: An Essence Drain can be revealed at the end of the Reset Phase, so that it will take effect during the Upkeep Phase.
• Immediately after a creature is activated, before it chooses its actions for the turn.
Example: You could reveal Chains of Agony when your opponent activates his creature. If the creature moves that turn, it will take damage.
• Immediately after a creature completes its move action, but before it takes a quick action.
Example: After a creature moves into a zone, but before it can make an attack, you could reveal Sacred Ground.
• At the end of any of the eight steps of an attack or three steps of casting a spell.
Example: After the Avoid Attack Step of an attack, you could reveal the Rhino Hide enchantment on
your creature to reduce the amount of damage it will take from that attack.
• You can reveal an enchantment immediately after it is cast, right after the Resolve Spell Step.
When an enchantment is “resolved” it is placed face down as a hidden enchantment. Then, immediately after it has resolved, you may choose to reveal it at the end of that Step.
• You cannot interrupt an event to reveal an enchantment.
Example: You cannot reveal an enchantment on a creature in the middle of its Move Action, or in the middle of rolling dice during an attack. You would have to wait until that “event” (step or action) has finished.

In this portion, we learn that events are steps and actions, and by implication, also phases. We learn that rolling dice is an event.

If a mandatory or single use enchantment is revealed at any time other than the event for which it is supposed to trigger, it has no effect and is immediately destroyed and discarded. You cannot hold the revealed spell, to use its effect at a later time. For example, if a Block spell is revealed when there is no attack, it has no effect and is destroyed and discarded instead.

Here we learn that events are anything for which an enchantment triggers. Which makes sense, because it allows all enchantments to be revealed after they trigger.

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I really don't see what the steps have to do with it at all. It's merely a question of what a reroll is and is it changing the past? If a reroll is changing the roll, then it has to happen when the roll happens because the game doesn't allow you to change the past. If it's not changing the past, and rather is a brand new occurrence, then it doesn't matter when it happens or when the enchantment is revealed as long as you can still make use of the dice somehow.

Let me put it as a question: would we have any reason to believe that the following card breaks the rules?
Re-roller. Mandatory Enchantment: Whenever this creature rolls an effect die for an attack and gets a result below 7, you must reveal Re-roller. Re-roll the attack and effect dice, then destroy Re-roller.

If we can do that, we have to be revealing the enchantment after the roll, because there's no way to reveal it during the roll. There's no reason to suspect that the roll procedure continues after the dice have stopped moving, is there? I propose that the only time the rules would allow a re-roll (if the above enchantment is legal, and it seems to me like it would be) is after the roll.

Under the version 3 rules, you could always reveal any enchantment after the roll, which would also be the soonest a mandatory enchantment could be revealed. This was because rolling dice is an event, and you could reveal after any event. If we believe the mandatory enchantment has an opportunity to function within the rules, all hidden re-roll enchantments could have done the same.

Under the version 4 rules, since you can't reveal after the roll unless the enchantment specifically tells you to, you have to wait till after the roll step to reveal Akiro's Favor. It's possible that once the step is passed, you're outside the window of opportunity for re-rolls, so you can't reveal in time to have any effect.

That's why steps come into it. Previously, you didn't have to wait till the end of a step to reveal, but now you do, and that may cause you to have to wait longer than is permissible.

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I like your supreme court of Mage Wars analogy. We don't know the outcome, just the existing law such as it is. The decision makes new law. 

I can imagine a novice spawnpoint with no channeling and some sort of every-other-round timer for pretty cheep. But I'd want a reason to do it. Making all mages more similar cuts off other options.

You mean Rhino Hide?

Ayep. That'll teach me to write on my 10min break.

here i would prefer if the official answer would treat the global problem (how far in the past an enchantment can have effect when it's revealed) and not only the A's favor problem. i prefere a unique global rull than 30 exceptions.

There is already an official answer to this question in the rulebook: time is never rewound. Temple doesn't "effect the past" either. A re-roll is never an undo, it's always a re-do.

Why does it have to be done within a step?

Maybe it doesn't. I don't know, because we don't have many rules for re-rolling. It's totally possible that you can re-roll any time before the roll is applied.

The old rules called rolling an event, so it was more clear that you *couldn't* re-roll during a roll, so you had to re-roll afterwards. Now? Donno.