Q: Can Acid Phlegm be used in melee combat?

A: No. Acid Phlegm is a "Fire of Opportunity" power. Under the rules for Fire of Opportunity (p. 58): "Fire of Opportunity must occur during the model’s Activation Phase, and cannot be made if the model enters melee combat."

So not only can it not be used while in Melee, it can't even be used on the same turn the Harbinger moves into melee combat.

Frenzied Warriors:

Passing and swooping attacks do not make base-to-base contact, so they do not count as melee combat. Therefore, if the target of a passing/swooping attack is the beneficiary of Frenzied Warriors, the benefits are not removed due to a passing/swooping attack made against them.

However, if the target is able to make a successful counter-attack (where allowed to do so), those benefits are lost.

If the model performing the passing/swooping attack is under the effects Frenzied Warriors (assuming such a thing is possible), they will always lose those benefits following a successful passing/swooping attack.

Blood Rage:

In addition to the extra bite attack granted, the wound that first triggers the Blood Rage grants the Harbinger of Sobek only a +1 ATT/-1 DEF (not +1/-1 for triggering the power, and another +1/-1 for that wound itself).