The length of the heat is something that one of the breeders will be able to answer. He is marking because of your female being in heat. I know you said that they are constantly supervised but
accidents can and do happen penetration/locking does not even have to happen
. The only way that you can be absolutely sure there is no pregnancy is to keep one locked in a crate at all times and alternate who is allowed out. There have even been stories where the male has broken out of the crate for an in season female. If he is allowed to mate with the female the marking could/will become even worse and many times there is also a personality change.
I would suggest that you also have your female spayed.