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The swarmlord's swarm leader ability to grant himself or a friendly unit preferred enemy etc, can be used even while he's engaged in an assault?

It seems to me like the rule allows for that, since he's not actually performing a shooting attack. Rather it just tells us when the ability becomes available.

In a similar way that psychic powers that buff/debuff can be used even if your psyker is engaged in an assault.

Yes.

We assume that if there are no restrictions on using powers or special rules in combat, you may do so. I can see where the confusion comes from, as most psychic powers that are used in the shooting phase (like all witchfire powers) cannot be used while in combat, but that's because it is explicitly stated in the rules for witchfires (or other powers) that they can't, since Swarm Leader has no such restriction, it can be used while in combat.