Do thumb-sucking and pacifiers ruin children's teeth and bites?

No. Thumbsucking and pacifier-sucking don't "ruin" teeth, but they will lead to more overbite in the baby's front teeth. Be sure these habits go away before the permanent teeth come. Throw out the pacifiers no later than when throwing out the bottles (around age 20-24 months). Since many or most kids get braces to straighten their teeth in the teen years, an overbite should be easily correctable.
Thumb is worse. Pacifier usually discontinued before child starts school, thumb/finger habit can persist. Has to do with intensity and duration. Pacifier is passive, thumb/fingers are active, pulling upper front teeth and the front of the upper jaw forward, collapsing the upper back arts of the jaw inward, and restraining normal lower jaw growth. By 3-4 you should be gently encouraging your child to quit.
No. Thumbsucking and pacifiers can cause changes in teeth alignment and bites, but this can be fixed. Many thumbsuckers cause their upper front teeth to push forward. Pacifiers tend to lead to an "open bite" meaning an open space between the upper and lower teeth, even with the jaws closed. Pacifiers can be taken away, while, thumbs, of course can not! talk to a pedi dentist or pediatrician for advice.
They can. Sucking habits can be fraught with many ill-effects upon the dentition, and yes as noted by a colleague they can be fixed, but in certain facial growth patterns, it can cause serious deflection from more normal vertical growth, a reduction in upper width and add to respiratory patterns that negatively add to the mix. So, yes sucking habits can be very bad, especially for a "poor grower".
Yes. These behaviors, if they continue past a year or so (especially, i find, over 2 years old) may be associated with teeth/jaw problems. Early on - especially by the 4-6 month range, try and help your baby develop other ways of self-soothing.
Yes. Yes they can. Although the pacifier or thumb may sooth the baby, there may be severe damage to the teeth ranging from an overbite to caries ( cavities). Best not to start the pacifier and hope the baby does not sucke their thumb as there is no totally successful method to stop thumbsucking. Try to find another safe soothing object.
Yes. Yes, but the comfort they provide may be worth it -- as an old mentor of mine once said, "an orthodontist is cheaper than a psychiatrist.".
No. For the most part no. Today's pacifiers are much improved over the original models as they strive to be more "orthodontically correct". The pediatric dentists in our area tolerate orthodontic pacifiers (nuk, mam, dr. Brown, avent, nuby, etc.) until age four. Thumb-sucking seems to be slightly more of a mixed bag. However, psychologically, it is oft argued that it does more good than harm.
No. Not unless thumb sucking and pacifier use are prolonged well past 2 years of age. Some infants have a very strong suck reflex and need to suck even when they are not hungry. The good thing about thumbs are babies can put them back in their mouths by themselves. The good thing about pacifiers is that it is easier to pull the plug than to cut off the thumb.
No. Most dentists agree that until age 4, thumbs and pacifiers do not cause any dental problems, as they are concerned with the secondary teeth, not baby teeth.
No. Many times you can tell early on if a child will need braces later. Family history, small mouth, crowding of teeth. I tell parents go ahead and save for braces now and if the child doesn't need them , then you have money for vacation or whatever else you desire.It is rare in my experience that thumb and pacifier re sole reason for dental issues.
No. They can. I have had only a few children in many years of practice that had dental issues directly from thumb sucking. Many children began thumb sucking in utero and it is something they will outgrow. Pacifiers can be life saving for the sanity of some mothers, and pacifiers can be removed by 18 months without causing dental problems.
Yes. There is truly a lot of variables in this question - i.e. How much does the child use a pacy of suck their thumb per day, but for the most part these habits are devastating to teeth. Not only do I see this every day in my office, but I have a child who sucks her thumb and see the devastation to her teeth every day.