Hi there. Great spell, I learned a lot from reading these triggers, but I have a few concerns.
First: I noticed your dummies can be raised/do decay. This has negative interaction in places, the first and foremost of which (aside from leaking corpses) is that the missiles persist for much longer than intended. You give them a 2-second lifespan, but because decaying units may still exist in groups, the "dead" missile continues flying until it completely decays. This led to a hilarious event wherein I accidentally scouted from one end of my map to the other with a missed missile. The fix is obviously to just change them to Can't Be Raised/Do Not Decay.
Second: Depending on the castable arc of a unit, your missile can end up being cast at funny/unintended angles. A simple fix for this appears to be to set the facing of the spawned dummy arrow at "facing point of ability being cast" rather than "facing of casting unit".
Third: Just a question, what does setting the Animation - Change (Last created unit) flying height to SA_MissileHeight at 1000000000.00 actually do? That's a huge number, and I haven't actually dealt with flying heights and stuff before, so it was odd to see one billion anythings.
Other than that, great spell, has performed flawlessly, and it's opened my eyes to yet another trick that can be used to make interesting spells. Thank you so much, great resource.