What is the evidence for the two formulas of salutation and peace upon the Chosen One, peace and blessings be upon him, namely: "عليه الصلاة والسلام" (upon him be peace and blessings) and "صلى الله عليه وسلم" (may God's peace and blessings be upon him)?

The best way to invoke blessings upon the Prophet, peace and blessings be upon him, is according to the manner narrated from him. As for the two aforementioned formulas and similar ones, they are permissible, and the predecessors of the Ummah and its scholars have consistently invoked blessings upon him using such forms.