The candidates may not get to see the spoilt papers. There is often someone checking for each party in election counts, who would. This being an election with large constituencies and many independent candidates, they may not all have sufficient people to do the checking, especially if counting is happening in more than one place.
I'm no expert on the law of libel, I don't know if you have to 'publish' something for it to apply, or just write it down and show other people. Or perhaps those 2 are legally the same? I guess the usual rule of thumb applies. If you have to ask someone else if something is a good idea, then it usually isn't.