If I were you , I would quit worrying about cancer in the rabbit. Pet type Vets. that may say that, are just out to liberate you from your money. Animal rescues will tell you anything to scare you in spending more money with them. Many do gooders are just plain ignorant and will parrot what ever they have heard. I have seen quite a few old female rabbits that never had a litter that were perfectly healthy in old age. A rabbit may or may not get cancer, it is totally out of your control. As for your resting board with " a thick layer of soft bedding" ... that is the root cause of the yucky stuff and red skin, etc. . Killing her with kindness. I would remove it and just leave the board only and change it when it becomes urine soaked. Just let the rabbit be a rabbit.