Can a dentist put a veneer over a crown?

It depends. Depending on the material used to make the initial crown, it can be successful if it is an all ceramic crown and relatively new. I've done it after a patient refused to have me remove their chipped all ceramic crown. They understood there was no guarantee but 8+ years later it still looks great. So, yes, it is possible but the dentist has to understand the limitations. A new crown is best.
I wouldn't. It is not easay for porcelain or composite to stick to the surface of an existing crown with out extreme heat that can not be attained in the oral cavity. Besides, the cost of the veneer is not much less than a crown so just have the crown replaced and you can have the look that you want and will last much longer.
Does not make sense. There is a time and money investment associated with placing a veneer. If you are undergoing this cost, why not replace the crown? The cost will be similar, it will appear much more aesthetic, and instead, there will be no weak interface between the veneer and the crown.
Yes. It is possible. It requires that the tooth be healthy (and not need to replace the crown instead). The results may not be as ideal as replacing the crown, because there may be limitations dictated by the shape or color of the crown, .
Why. Not sure why you would place a veneer over a crown. Veneers are bonded into place and bonding porcelain to porcelain is not as predictable as some other ways. I would just recrown the tooth.
Sometimes. There should be a good reason for not simply remaking the crown, which is often the best solution. If the crown was just placed, and the rest of the smile was to be veneered, then yes, it is possible, especially if the crown was all porcelain (no metal). The bond can be very strong if done right. Normally it is just better to simply make a new crown.
No. It is either one or the other. Placing a veneer over a crown probably will not last long.
Generally no. If there is a problem with a crown (either the aesthetics or the fit), then that must be addressed first. To get a great results on a front tooth, visit a cosmetic dentist. Discuss your concerns and what you hope for in the final result. They can direct you to what the best method of treatment would be.
Not really. Crown and veneer are different veneer usually cover the surface with minimal prepping crown is more aggressive go around the hole tooth placing a veneer over a crown is not doable.