I do not follow you...how can you positively conclude that there is already a devil before earth's creation week.
Because he is immediately there in Genesis 3 - and is already evil.
Rev 12 says he is "the serpent of old" the devil who drew away 1/3 of the angels of heaven.
Isaiah 14 and Ezek 28 both say he was created as Lucifer - sinless , perfect, "until the day" that iniquity was found in him. And the language implies something more than a few seconds or minutes of time for his fall, ,for this appeal to 1/3 of the angels and then to his losing the war and being cast down to Earth.
In the beginning there is only God...all else is void.
But then God chooses to "create the heavens and the Earth" Genesis 1:1 where the Creation of earth into a planet with life on it - starts off sometime afterward so that Earth is already solid rock covered by water before day 1 of creation week.
Trying to compress the creation of the perfect angel Lucifer - covering Cherub who is on the mountain of God, walks amidst the stones of fire - perfect and wise "until the day" that iniquity is found in him. Sound more like years/decades/centuries than like "seconds or minutes". And makes more sense that way.
Furthermore for him to argue the angels (1/3 of them) to be on his side then have war in heaven, then lose that war ... also sound more like months/years than like minutes/seconds.
Here is the explanation of Lucifer's fall:
Isaiah 14:12-14:
2 How art thou fallen from heaven,
O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!
13 For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
14 I will ascend above the heights of the clouds; I will be like the most High.
Ezek 28
“
On the day that you were created
They were prepared.
14 “You were
the anointed cherub who covers,
And I placed you
there.
You were on the holy mountain of God;
You walked in the midst of the stones of fire.
15 “
You were blameless in your ways
From the day you were created
Until unrighteousness was found in you.
16 “By the abundance of your trade
You were internally filled with violence,
And you sinned;
Therefore I have
cast you as profane
From the mountain of God.
And I have destroyed you,
O covering cherub,
From the midst of the stones of fire.
17 “Your heart was lifted up because of your beauty;
You corrupted your wisdom by reason of your splendor.
I cast you to the ground;
I put you before kings,
That they may see you.
18 “By the multitude of your iniquities,
In the unrighteousness of your trade
You profaned your sanctuaries.
Therefore I have brought fire from the midst of you;
Rev 12 - he is the devil who draws away 1/3 of the angels and is the "serpent of old" the snake in the garden of Eden. Already fallen --- already evil , in Eden.