I'm ok with Lucas for the most part.
He sees Billy firing his gun from the corner of the little house and has the fight ending when Billy is disabled. Fits in well enough with the rest of the testimony.
The part about having his gun in the air is questionable.
This render is taken from the second story of Girds block at the door closest to 4th street.
Certainly less than exacting, but I think it makes the point that Lucas' view could easily be blocked. In this pic Virgil and Wyatt can't be seen because they are in the vacant lot. The views of the others are blocked because of awning posts, railing, and in the real world who knows what else. (Frank, the horse, and Doc can just barely be seen on the far right.)
https://dl.dropboxusercontent.com/s/amvqxqftxsf8q9i/OK_View_Lucas.png?dl=0
If his view was from one of the doors farther west, then the view changes and I see no reason he shouldn't have seen more of the combatants.
Jones