All drains in your house should have elbows on them to prevent smells from coming back up the drains. If your drains smell make sure they are all installed correctly.
More than likely there is something blocking the lines causing them not to flow freely. Try some of the chemical drain cleaners, but more than likely it is best to get a plumber to come work on them to gaurantee a result.
Backup sewage would definitely create a horrible smell. You would however hopefully see the backup. It could also be cause by a break in the pipe or by a floor drain or trap that has evaporated. GO through the house and poor a gallon of water down each drain.
It depends on which pipes. It is possible, but not likely the cause. You may have a buildup of hair or something that could be solved by using pipe cleaner.
most likely no but there might be a problem with the traps that are being used for your lavatories and that they might need to be re piped to stop the smell.
