Q: The current FAQ states that 'Heroes are never immune to a final Avatar's attack.' Does this mean that Heroes cannot be immune to ANY aspect of the Avatar's attack (e.g. is a hero with Ironskin immune to the Sorcerer King's Sorcery ability? Is a hero with Ox Tattoo immune to the Spider Queen's Web ability on her attack?)?
A1: Heroes can be immune to abilities of Avatars; they just cannot nullify the attack dice themselves.
A2: A hero can never be immune to ANY aspect of an Avatar's attack.

Secondary question
If a hero is immune to a Lts attack, then that can make it impossible for the Lt to defeat the hero party meaning that the Heroes can either automatically break seiges or a long and impossible to resolve impasse can be reached during an encounter. This may or may not strongly affect game balance.
For example, Zyla (Ghost) is immune to all attacks by Darkwind and all his minions. So is any hero with Ox Tattoo vs the Kraken.
Should the current FAQ question (and the above one) add "...or Lieutenant..." after Avatar?

Any objections to moving this to the FPD as is?

Using it as a houserule has been critical for my SoB campaign. Without =out Darkwind cannot possibly kill Zyla and so as long as the heroes can acnhor the Revenge, they are guaranteed to force Darkwind to flee. It just might take 10 years to lay out the encounter as Zyla can barely do a single point of damage per attack on a good roll with spending extra fatigue (but can rest at will as she is entirely immune to all monsters in the encounter). It takes about 4-5 turn to get in each attack....