So, Garling gets the credit for killing Xebec and whoever the SC was before him gets executed for his failure, allowing Garling to get promoted.
Oh you mean like: what if Xebec fought and killed the former Supreme Commander instead, and Garling only came in at the end and killed him so to become the new Supreme Commander.
That would've been a good alternative too ngl, but I guess that's already to many characters to handle for Oda in a flashback.
Also, quickly killing a newly introduced Supreme Commander wouldn't really have given a real demonstration of Xebec's strenght imo, on the other hand Garling was one of the strongest fighters in God Valley, so Xebec defeating him properly hyped Xebec up as the strongest.
Btw I doubt Garling became Supreme Commander after killing Xebec, he was still to young.