The first two questions the answer would appear to be yes judging by the following in the Core Rules Designer's Commentary
Q: If I add units to my army after a battle has started, and
my army has an allegiance ability that adds a keyword to the
units in the army, is that keyword received by eligible units I
add to my army after the battle has begun? For example, if I
have a Stormcast Eternals army and use the Stormhosts rule to
give all Stormcast Eternals units in the army the Hammers
of Sigmar keyword, would any new Stormcast Eternals units
that I add to my army get the keyword? By the same token,
if an allegiance ability has a spell lore that grants a spell to
Wizards in an army, do Wizards that I add to the army
that have the appropriate allegiance gain a spell?
A: Yes to all questions.
As for the overlapping auras from Bray Shamans I would say we probably need a more specific answer on that but the below definition makes me think that if every model is covered it'd be valid even if by multiple Shamans
Q: When the word ‘any’ is used in the criteria for an ability,
how many times is that ability applied when the criteria for
the ability are fulfilled? For example, if an ability said ‘Add
1 to hit rolls for models that are within 6" of any models with
this ability’, would I add 1 to the hit rolls of a model that was
within 6" of three models with the ability, or would I add 3 to
the hit rolls?
A: The word ‘any’ is treated as being synonymous with
‘one or more’. In your example, this means that 1 would
be added to the hit rolls, not 3.
