As to your question about a kettle in the cabin - on my recent cruise aboard Rhapsody Of The Seas they had electric tea kettles for heating water in the cabins. This was a remnant from that ship's recent use for Asian cruises where the passengers wanted to make tea in the cabin. I cannot say for certain if this will happen on your cruise. These were also just for heating water for pouring into cups, so I am not sure how successful you would be in heating bottles of formula with it.
Personal appliances such as a kettle or hot plate are not allowed.