Hi Roma,
Umbilical hernia is hereditary, no doubt about it. It is however so common in this breed that the kennel club is accepting it as being normal or at least not discriminating against it. this is why you have recieved different opinions about it.
Some will tell you that the parents do not have it and the rest of the litter was normal so it must be for some other reason. further, some may say the pup was completely normal when you got it.
Well the hernia could not be felt but it was there and only became bigger as the pup grown. The reason why it could not be felt is beacause the fat in the belly did not protrude through the hole at that time, and as the pup grew the hole became bigger and the fet in the abdomen heavyer. It can also skeep generations.
If it is small it dose not need correction and has no health implications. your dog is going to be perfectliy fine with it. many peple will elect to correct the hernia at a time of castration or any other surgical procedure. Other will let it be.
Hope this helps. let me know about the other issues you had problems withI may have an idea.