I wonder if Coppard (or the Norfolk singer) misheard "room" and thought it was some mysterious "ru." Sounds unlikely I know.

The booklet accompanying Lloyd & MacColl's "Great British Ballads Not Found in the Child Collection" (1956) clearly prints "in the room of a swan."

Unfortunately I have no turntable so I can't verify what Lloyd actually sang.

The meaning of "ruse of a swan" is less clear than it might seem: "ruse" seems never to have meant what it would have to mean here, namely something like "assumed shape" or even "disguise." It usually means a "trick, strategem, or wile." A disguise may be a kind of ruse, but a ruse isn't a kind of disguise.

Again, the singer may have been trying to rationalize an unfamiliar "room of a swan."