Can a .csv spreadsheet be uploaded into Canvas gradebook seamlessly, even when there are missing or invalid IDs?

Yes, it can.  Canvas will prompt about the missing and invalid users but will still upload the grades that have appropriate IDs.

Required columns: Student, ID, SIS User ID, SIS Login ID, Existing Assignment (###), A New Assignment (if you want to create a new one in the new file outside of Canvas)

See also the Canvas Guide, How do I upload changes to the Gradebook?