In any electoral system, there is always a possibility of fraud. For example, I believe it is possible in the USA to walk up to the voting booth, say "I am Joe Smith of 123 Main Street", and get a ballot for Joe Smith of 123 Main Street if that name has not been crossed off.

Does this happen? Probably.

Does it happen to affect the result of the election? Probably not.

The examples given here seem to be anecdotes of a zealous party worker affecting the count count to less than 20 votes. For sure, that is not going to overturn a swing state in the electoral college. And yes, the party worker should be put in front of a judge. And it seems like that is happening.

If the parties start doing this en masse, that means an orchestrated effort that will be hard to keep secret. But even then, the party might garner 0.5% swing (which could flip the EC in some states). But if the system can find and prosecute a party worker with 20 votes, finding orchestrated party cheating should be easy.

The best way to prevent cheating at the polls is to have a competent candidate and an election campaign that the result is so clear that any alleged cheating is likely not to have an effect.