This right can be licensed
There are a number of copyright holders, but most uses can be licensed through Warner Bros. Television.
They do, if they're not licensed.
There are a large number of copyright holders; the song itself has seven but can generally be licensed through Warner Chappell, and the sound recording has at least as many but can generally be licensed through Atlantic.
Copyright is the exclusive right to copy, alter, distribute, or perform/display a work. For books, copyright is usually ascribed to the author and subsequently licensed to a publisher.
The copyright holder is EMI Unart, but most uses can be licensed through Alfred.
Patrick Marber personally is the copyright holder; performances are licensed through Samuel French UK.
A copyright is granted to the person that created it. A user name is not considered a proper identification of a person. You will have to use your legal name to register the copyright.
"Assume"? If you mean "inherit", then yes. A copyright is a personal property right that can be donated, sold, bought, gifted, bartered, inherited, licensed, etc.
Most Tchelitchev works can be licensed through Art Resource, linked below.
Dennis the Menace can be licensed through King Features at the link below.
Nothing will happen to a person who breaches copyright law unless the holder of the copyright chooses to sue the violator. The lawsuit court settlement will determine the monetary damagesowed to the copyright holder (if any).
Much of it would be licensed through other organizations rather than directly. Songs may be licensed through their respective publishers, and recordings can be licensed through the labels.