As mentioned under Mosiah 8:17, the correct reading here in Mormon 9:15 is passed, the reading in the LDS text since 1920. In this case, we have an instance of the present perfect for the verb pass: “have all these things passed”. If past (the original spelling) were correct, then we would expect the finite verb form are rather than have (“are all these things past”). Instances of past with the meaning ‘over’ are found elsewhere in the text; for discussion, see under Mormon 2:15. The critical text will maintain the past participle form passed here in Mormon 9:15.
Summary: Maintain the spelling passed in Mormon 9:15 since the finite verb here is the perfect auxiliary have (“have all these things passed”).