What really happened to Mr. Ewell and how did Heck Tate alter the evidence to fit his lie? Lots of detail please.
I think Lee leads us to believe that Mr. Ewell was killed by Boo Radley. The order of sounds that were heard by Scout confirm this. First she feels herself attacked, then Jem in a fight, a crack (the breaking of Jem's arm), and then a scuffle, and finally some short quick steps away. We assume the scuffle was a fight between Bob and Boo, and the short quick steps away were Boo taking Jem to the house.
The evidence shows that he had a knife stuck up under his ribs. We know this when Scout is in the dark and feels across his body trying to figure out who that is. Scout finds something she can't identify (a knife), and later Tate reports he found Bob under the tree with a knife stuck under his ribs.
Tate says Bob must have fell on his knife. (This is interesting because it would be odd for a person to purposely kill themselves this way, so falling was the only way this would have worked)
It was too dark to see anything, and Ewell easily could have tripped on a root under this tree.