The overly literal answer
The card mostly gives us this to go on:
Creature cards in graveyards can attack and block as though they were on the battlefield
Rosewater's FAQ specifies a lot of details, but a common thread stands out: creature cards in graveyards are only treated as creatures on the battlefield while attacking or blocking.
Over My Dead Bodies allows you to attack or block with creature cards in your graveyard. If you do [emphasis added], for the duration of combat, the attacking and blocking creature cards are treated as if they are creature cards on the battlefield which means they can be affected by spells or abilities that would affect creatures on the battlefield.
Remember you treat them as on the battlefield only while they're attacking or blocking.
Remember that they're treated as permanents on the battlefield while in combat [emphasis added], so a graveyard creature attacking will cause anything that triggers when a creature attacks (including its own abilities) to trigger.
What does this mean in practice? Well...
Combat begins with the beginning of combat phase. No creatures are attacking or blocking ("in combat") at this time.
Raving Dead's triggered ability "at the beginning of combat" will not trigger, because it is not actually in combat at this time.
Likewise, because Brutal Hordechief is not in combat, either, its activated ability cannot be activated at this time.
We proceed to the declare attackers phase. All attackers are declared simultaneously, then we check what state-based actions or triggers will resolve.
The card text itself says we can declare a card in a graveyard as an attacker. Rosewater's FAQ clarifies that this means it's virtually "on the battlefield" until end of combat. This is a continuous effect (c.f. the bit about how removing Over My Dead Bodies will instantaneously end it); thus, when we check for triggers after attackers are declared, all graveyard-creatures will see each other as "on the battlefield."
- Brutal Hordechief's "whenever a creature you control attacks" ability will trigger, and it will "see" every attacking creature, living and dead.
From this point until the end of the combat step, every attacking graveyard-creature is in combat, and therefore on the battlefield. None of the cards in the defender's graveyard are in combat yet, however, so none of them are on the battlefield.
- Based on the implications in the FAQ (treat creatures as really, truly on the battlefield), you may now use Brutal Hordechief's activated ability now.
The defender declares blockers. This is pretty symmetrical with the declare-attackers situation.
- Brutal Hordechief's activated ability will let you force your opponent to block with cards in their graveyard (respecting the constraint that the living may only block the dead and vice versa). I think it's a bit of a confusing chicken-or-egg situation but the most sensible and intended reading is clearly "if it could be a creature that blocks, use the normal rules to figure out how it blocks, and then it becomes a creature if it has now joined combat."
Damage is exchanged, then state-based effects and triggers happen as normal.
- At this point, Raving Dead is still in combat (and therefore, on the battlefield), and it just did damage to a player, so its triggered ability will trigger. Simple.
So, 3 out of 4 abilities worked as intended, but the quirk about how you have to attack or block before your creature is "animated" means that beginning-of-combat triggers don't apply.
I think it's pretty clear from Rosewater's FAQ answers that the intent of the card is to let you throw your graveyards at each other in combat, and the fun thing about it is really, really treating them like they're on the battlefield for the duration of combat.
I also think that, comparing the card text to the FAQ, it's clear that they struggled with a way to actually communicate that idea in Magic-legalese. Which is why the card actually says something very limited and most of the details about "you can use all your abilities and static effects apply and it's AWESOME!" are just in the FAQ.
In that light, I think it's reasonable to treat the Un-FAQ as not-quite-absolute, also. Just his best effort to communicate his vision using limited space and time, and likely only addressing the specific cases that came up during WotC's own design and playtesting.
If you feel like the best way to make sense of the card is to drop all these details about beginning of combat vs. declare attackers and just throw your graveyards at each other, I think it's reasonable to bend the normal flow of play and just say that abilities that technically trigger before a creature is "in combat" can still be used, as long as you triple-extra-promise that creature is actually going to attack or block later.
Basically, choose flavor-logic over rules-logic, if practical, because the rules-logic isn't going to hold perfectly anyway, and some of the corner cases that fall out (e.g. removing the randomness from Raving Dead) may actually end up rather unfun anyway.
(And if someone puts a Blazing Archon in their graveyard and says "now you can't attack me at all, MWAHAHA," go back to that overly literal section for the duration of that single argument.)