You have to demonstrate the consquence of any processes gaps or violation.Forgetting to stamp or sign here doesn't make process unfair and worse when effect of the results are such that it benefit the petitioner.
The first part is correct. One would indeed also have to show that any errors or mischief would have been sufficient to give a different outcome.
The second part is hopelessly confused. "Stamps" and "signatures" do not matter a great deal, and not just in elections. At the simplest level, that is so because processes---in modern life, generally---have been arranged to that "stamps" and "signatures" do matter a great deal.
It is not easy to determine whether a "stamp"/"signature" was simply "forgotten" or "not available" (as would be the case with deliberate mischief). So, without being able to look into minds, the default assumption must be the latter, and anything not properly "stamped" or "signed" must be invalidated.
Of course, forgeries or any other form of fraud would make the elections-process unfair.