Is it possible to change the place of the adjectives and the frequency adverbs? For example, is it possible to say both itket harvoin and harvoin itket?
Adverbials can usually be moved around pretty freely within the clause as long they don't split a clause element, but adjectives are not free to move around within their phrase except when swapping places with neighbouring adjectives.
Doeavthia mean that harvoin itket should be accepted?
Would "Te itkette harvoin, ja nauratte usein." be a possible translation? i.e. taking "you" to be plural?
Yes, but I don't think the comma is necessary. The oxford comma is not a particularly well-established thing in Finnish grammar.
Yeah, plural should be an acceptable translation. Remember to report if both are not accepted!
"You rarely cry and laugh often" sounds more natural in English