Long story short, it doesn't matter if Total_CP_Multiplier comes from CP_Multiplier or Additional_CP_Multiplier.
Option 2 is more correct, but it really doesn't make a difference what level you capture it. While the game tracks the difference, in practice powering up is exactly equivalent to capturing at a higher level in the first place. Additional_CP_Multiplier stacks with CP_Multiplier as if CP_Multiplier just kept going up the list in half steps.
You can think of it more like the game tracks capture level and power up levels, and when it needs to use CP_Multiplier it combines the levels and checks the table to see what value to use.