She grinned happily.
She happily grinned.
Same words. I generally prefer the later, but they have very slight differences in emphasis. Without the adverb, the subtle difference is lost.
She happily grinned.
Same words. I generally prefer the later, but they have very slight differences in emphasis. Without the adverb, the subtle difference is lost.
Moving "only" to any other spot in this sentence is grammatically correct and changes the meaning.
As for "rules", well, welcome to English: the rules are made up and the points don't matter.