Q1: The right form of interrogative should be "Can outfit represent cancanonicals?".
"cancanonicals" is one of the Ecclesiastical Terms. It means "the dress prescribed by canon for officiating clergy".
"outfit" menas "a set of clothes that you wear together, especially for a particular occasion or purpose".
In my opinion, outfit can represent cancanonicals. But I am not quite sure. I hope there will be other people who can answer this question.
Q2: If it really mean "you two are close", I think they say like that deliberately. In fact, serious and cloase can be seen as a pair of words with opposite meanings. So in fact, the narrators want to say "you two are close", but they use a opposite word to add to a kind of humorous atmosphere.
这个跟中文里面情况应该是一样的,为了达到一种搞笑的效果,故意说反话。但是这个serious如果在正式场合还是严肃的意思。
Q3: The difference between jewelry(jewellery) and jewel is similar to that between machine and machinery.
The jewellery is "objects such rings and necklaces that people wear as decoration and they can be made from every kinds of materials like diamond, gold, silver"; while jewel mainly means "a precious stone such as diamond; also jewellery or other objects made with valuable stones and used for decoration". In other words, jewellery includes jewel.
Q4: The basic structure of this sentence should be "The exhibit was a addition to the museum." It means "展览是对博物馆的一种补充(形式)".