Question

what three pieces of evidence or testimony provided either in Leschi's trials, gathered for the appeals process or in the effort to gain support for Leschi to be pardoned provided the strongest argument for saving Leschi

Answer 1

There is a detailed examination of his case at this link 
http://www.washingtonhistoryonline.org/leschi/

Ive not read all of it myself but would expect the answer to your question to be there. 

Unfortunately the "quashing" of his sentence came far too late to save him from being hung