Sounds like there is a lot of confusion in this topic and it's confusing to go through. Need to break everything down into components and be specific on which one your talking about
1. Can bicep guy legally carry a gun?
2. Is bicep guy justified in attacking Kyle?
3. Is Kyle justified in shooting bicep guy?
Regarding an airsoft pistol:
For 1 - If the assumption is bicep guy couldn't posses a gun, then the prosecution would need to prove the gun was real as an element of a crime ie. Felon with a gun or something. The defense stating it was an airsoft could be reasonable doubt to win the case. If the prosecution had video of the gun shooting and wounding someone, then they could prove their case without having the gun.
For 2 - an example is an attempted murder charge on bicep guy. Let's assume all other elements are met, the prosecution would need to prove the gun is real as he would have the means to commit the crime, an element needed to convict. If the prosecutors didn't have the gun, the defense could say it was an airsoft therefore he did not have the means to murder Kyle.
For 3 - The prosecutors focus is on Kyle and they would need to prove his gun is real and that he shot bicep guy. Prosecutors can prove that with video, witnesses, and forensics without needing the rifle. On Kyle's side, they don't need to prove bicep guys handgun is real. For a self defense claim they need to state Kyle felt threatened by it, that he saw what appeared to be a gun and that he was afraid it would be used on him.
Bicep guy and skateboard guy actions could also be seen as self-defense of others. They see Kyle shooting baldy and are afraid Kyle will shoot others so they attempt to bring Kyle down.