Onnea means congratulations, right? The source does not say "many congratulations." So the answer should not either.
"Paljon onnea! " is a Finnish idiom for " congratulations" That is what you normally say, not just "Onnea" Saying just "Onnea" is not wrong, but "Paljon Onnea" is more natural, polite and cheerful.
Yes Onnea alone is congratulations, the paljon is unescessary here
Yes, onnea should be accepted, reported.
Please, add Onnea without paljon to correct answers. Reported
But cake is a lie
Still not accepted.