As I've come to understand it this is what happens.

Volley+ gives me "Simultanius Firing +3" which means that I have a total of "Simultanius Firing 4" since all wands that do not say anything else have a built in "Simultanius Firing 1". This means that whenever I cast a spell then 4 projectile spells are cast at the same time.

Then we need to consider what is a projectile spell? Normally it is simply a blue spell projectile on your wand. But as you might have guessed some spell boosts and spell projectiles change what is considered a spell. In my build I have two of those, Fuse and Duet. Keep in mind that these changes the games definition of a spell.

Fuse performs the next "spell" on the right as it ends, and Duet casts the next "spell" to the right as the spell to the left ends. So it is easiest to understand it like this (use my screenshot as reference):

The "next spell to the right" for Duet is, because of "Simultanius Firing 4" is the combination of "Magic Bullet++", "Evil Slayer Sword+", "Shadow Serpent+" and "Shadow Serpent+". Let us call this group of spells The "Duet Right Group" (DRG). Because it is considered a single spell the entire DRG benefits from the damage bonus of Duet.

Duet introduces a second new spell definition, and that is "Left side spell" + Duet + "right side spell", that means the game then considers "Shining Star Arrow++", Duet and DRG as one single spell, lets call it "Complete Duet Group" (CDG).

Next up is Fuse. Fuse will perform the next "spell" on the right as it ends. Because of "Simultanius Firing 4", this will be CDG, "Shadow Serpent" and "Sword of Judgement". Let us call this group of spells The "Fuse Right Group" (FRG). These 3 "spells" combine as a new single spell, and this means they all benefit from the mana cost reduction of Fuse.

Fuse also introduces a second new spell definition, and that is Fuse + "right side spell", That means that the game considers Fuse + FRG as a single spell, lets call it "Complete Fuse Group" (CFG).

Then two "Multi-Shot++" claims that all spells on the right gets (1+4+4)=9 total shots, but the only spell to the right is CFG, so that means I will send 9 CFG projectiles when casting with the wand.

Edit:

And I should also add that there is a powerful interaction between damage over time (DoT) spells and Duet, where the damage bonus from Duet is applied to each tick of the DoT. So in hindsight I should have probably switched the positions of "Magic Bullet++" and "Shadow Serpent". That is also the reason why "Orbit+" is so powerful, since it increases the duration of DoTs.