Having never used parallel, I still believe parallel was correct.

./Ham' 'Jam' 'Spam

would be identical to ./Ham\nJam\nSpam (if \n were the correct translation to the newline in this case) or './Ham Jam Spam'

This would be identical to what you wrote, but only punts to quotes when it doesn't have a canonical method of representing the character otherwise. The fact that you don't need to explicitly concatenate two strings in the shell may be what's throwing you off?

Interestingly enough, 'Ham\n\nJam\nSpam' becomes

./Ham' '' 'Jam' 'Spam

So parallel is just literally outputting all newlines using quotes. I believe this would be identical, if you analyzed it and saw that two newlines are next to each other:

./Ham'

'Spam' 'Jam