For a mock trial, what strong points or evidence could be used to make Jem seem as if he is guilty of the murder of Bob Ewell, and not Boo Radley...

Part of what Scout has told Sheriff Tate, along with the condition of her costume, may help to implicate Jem in a mock trial after Bob Ewell is killed.

When Heck Tate questions Scout about the death of Bob Ewell, she tells him what happened prior to Ewell's death. During her recounting of the events, Scout says that she and Jem heard someone, but they just thought it was Cecil Jacobs, who had already scared...

Part of what Scout has told Sheriff Tate, along with the condition of her costume, may help to implicate Jem in a mock trial after Bob Ewell is killed.

When Heck Tate questions Scout about the death of Bob Ewell, she tells him what happened prior to Ewell's death. During her recounting of the events, Scout says that she and Jem heard someone, but they just thought it was Cecil Jacobs, who had already scared them that night. 
The person who knocked Scout down was not Cecil, however. Jem yelled, and she heard footsteps that stopped whenever she and Jem stopped.

"Then all of a sudden somethin' grabbed me an' mashed my costume....heard a tusslin' under the tree sort of...they were bammin' against the trunk.... Jem found me and started pullin' me toward the road....Mr. Ewell yanked him down, I reckon. They tussled some more and then there was this funny noise--Jem hollered...
"Anyway, Jem hollered and I didn't hear him any more and the next thing...Mr. Ewell was trying to squeeze me to death...Then somebody yanked Mr. Ewell down. Jem must have got up. That's all I know." (Ch.29)

From Scout's recall of the events, she believes that Jem must have jumped on Mr. Ewell while he was on top of her. 
Judging from the appearance of the frame of Scout's costume that is severely bent, and the "shiny, clean line" that stands out on the dull wire, Ewell meant to harm or even kill Scout because he must have had a knife or something similar.
Also, that Jem wrestled with Ewell while he was on Scout is apparent from the little holes that were in Jem's sleeves and the puncture marks on his arms to match the holes.