I am playing solo with the demo deck while waiting for my core set to arrive. Lannister vs Stark joust. Lannisters reveal Wildfire Assault plot card. Eddard Stark (stalwart) and Benjen Stark (shuffle dead pile back into deck upon death) are among those sacrificed by the Starks. It seems to me that it would go like this: The multiple killed characters due to wildfire go to the dead pile, ned is plucked out and placed on top of deck, but then loses his top of the deck status when the other dead are shuffled in. Is that correct? Please help me learn with actual rules or FAQ citations if possible, thanks.

On a side note, I am looking for a tutor or other noob to learn with via OCTGN/Skype. PM here or Eric_B on Skype.

You're thinking of it logically, but you're missing a key component; The Moribund state (elaborated in the faq)

When characters are killed, they aren't actually placed in the dead pile until the end of the action window (until all passives/responses have finished). Eddard Stark dies, but won't hit the top of the deck until Step 6 of the Action Window, and Benjen won't hit the deck until Step 6 either.

Wildfire is revealed (1)
Opportunity to cancel (Step 2)
Wildfire is resolved, all but 3 characters die for each side. (Step 3)
Passives to any of the above are triggered. This includes Benjen's "passive", so each player's dead pile is shuffled into their decks.
Responses to any of the above are triggered. (Step 5)
Action window closes; all characters killed in steps 1-5 hit the dead pile or, in the case of Eddard, the top of your deck.

The general version of that is:

No card that leaves play will be physically put in its out-of-play area until all related passives and responses are fully resolved.

The shorthand specifically for Benjen's ability is:

Because of the timing of physically removing cards from play, Benjen's "shuffle your dead pile" effect will never shuffle Benjen himself (or any other characters killed at the same time) back into your deck.

I've only recently learned that all the plot When Reveals occur in the same window - i.e. cards are in moribund for the all the when reveals ... I take it that is correct?

i.e. One player reveals Valaar and a religious character is killed. Another played reveals Many Powers Long Asleep but cannot return that religious character as they are still moribund

I am less certain about plot phase responses i.e. Valaar is played and a bouncing Theon is killed

Response: After a unique character is discarded from the top of an opponent's deck, put Theon Greyjoy into play from your dead pile"

I also have Frostfang Peaks

Response: After you reveal a plot card, discard the top card of each opponent's deck

Are plot related responses played before or after When Revealed?

What about passive plot phase things - i.e. Golden Tooth Mines - Each time you reveal a plot card, draw a card.

If after, would all the cards be in moribund and hence Theon could not be returned (assuming a unique character was discarded due to Frostang)

Plots are Revealed.

When Revealed Plots are resolved in the order determined by the first player.

4. Passives Trigger (eg. Golden Tooth Mines)
5. Responses Trigger (eg Frostfang Peaks)

6. Moribund cards leave play, action window closes.

So Theon won't come back from Frostfang Peaks, for the reason you state.