Thumb sucking is a normal reflex in young children. However, thumb sucking after the age of 4 or using a pacifier after 6 months of age can change the shape of your child's growing jaw bones and interfere with the proper eruption of permanent teeth, so it should be discouraged. Try spreading crushed garlic on your child's thumb for a few days, or — if you prefer a rewards system — keep a star-chart for thumb-free days with a reward after 10 stars.
You should call your dentist. Sometimes it's necessary to remove primary teeth to make room for permanent teeth. But don't try to remove the baby tooth yourself.
Yes. Children need their baby teeth until they are 12 or 13 years old. Untreated decay could continue to eat away at the tooth until it reaches the pulp or nerve and cause extreme pain and/or infection. If baby teeth decay to the point of needing extraction, these missing teeth will interfere with your baby's ability to learn to talk properly and the proper spacing and alignment when adult teeth come in.