Painting on canvas and installing is always better since it can be removed and placed elsewhere, as of improvement of the house value... it is the metter of opinion, but you can always tell the client that since it is removable therefore it is collectable and it is rather investment than improvement. when it is painted directly on the wall it is just a decoration which is a matter of taste (one person likes the other one does not)

yes, you can use the same glue.