Delay validation should be set to True, not False, if the source file does not exist until run-time.
You could approach this a different way. Set up a hard-coded connection to, say, Data.xls and as the first part in your package, copy the dynamically named Excel file to it, overwriting what is already there.
Then allow the process to run using data.xls, which will always exist and your validation problems should go away.
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