OK, here is the flow chart with pretty colours and all. Version 1.0.
The idea is that when the rules refer to "When you declare an attack against...", they are referring to what I here call the "Initiation phase" as a whole, marked as the green outline on the top. This phase consists of two steps - the Declare/Trigger step, and the Target step.
During Declare Action, you pay fatigue costs, and therefore you also upgrade to power attack. This also explains why Power Attack is listed as its own action in the rulebook - it's more like an alternative attack declaration than an upgrade. And only available in the box marked as "Declare attack action".
Noteworthy: Leo's Pounce triggers after Declare, and resolves before Target.
During Choose Target, you choose the target. This is where Aegis, Taunt and Fade kick in. Since Spite has to go through this "Choose target" step, Walrus is right that Spite logically has to pay the cost for Aegis, even though it feels weird to me. The reactions do not go through this step.
If we want Spite to not have to pay for Aegis, Spite would have to have its own "Choose Spite target" step that would be different from "Choose attack target". But I don't see any basis for such a reading as I read the rules.
(You could also just remove the two Target boxes after the reactions, and argue that the reactions skip the Targeting step entirely.)
Once you have Declared/Triggered and Targeted, the Initiation phase is over, and you move on to the Resolution phase, marked as the blue outline at the bottom. Which consists of two steps: Roll combat, and Apply damage.
As Walrus pointed out, there are three different types of combat that roll dice in their own way. All three types of combat include a pre-rolling step though, which is Use Favor, where both attacker and defender get to add Favor dice. Once the dice have been rolled, there are always two possible outcomes: No Success, or Success.
Normally, only the attacker has the option of being successful, but unique to the Counterattack Combat Roll, in this roll the defender can be successful too.
If the result is No Success, then the flowchart ends. If the result is Success, for either attacker or defender, we immediately move on to the grand finale, the shared step of Apply Successful Attack Damage.
As I understand it here, any time the rules say "Immediately after a successful attack...", they refer to a timing point at the start of the Apply Damage step, before damage tree boxes are selected. (Edit: Basically during or immediately prior to step 1 of the damage application sequence I posted in the following post.)
If I am correct in this understanding, then Sereqet would be able to use Envenom any time after she has made a successful combat roll, regardless of whether her successes originated from an attack, a counterattack or an opportunity attack. This feels right to me, personally.
Other types of attack, which aren't included in the flowchart, but could be, would be Wild attacks and Living Hazard attacks.
Does this all make sense to you, Walrus?