What if Initiative is not Speed. If everything is happening "simultaneously" then Initiative is the action that has the strongest influence (among many influencing factors, it just happens to be the largest influence see plurality).
The Guy who acts first in the order of initiative Influences everyone's actions and options.
If I try to make it an element of Sequence - it contradicts simultaneity - the initiative is the Actor and the Actions that shape the initial conditions.
Everything is happening Simultaneously - but Some Actions influence Others more.
What happens is that that as the Player Converts/Transform the Conditions into his Actions the GM determines the Difficulty and Modifiers for the Roll, and then considers the effects.