Sometimes the impression can get distorted or ruined at the lab before the veneers are made. Your teeth have already been prepped, so although the temporaries will have to be removed, your teeth shouldn't have to get drilled again.
Ten years is a very safe bet. As auth anything, maintenance is important. With proper home care and regular dental check ups, veneers can last a very long time.
Matching a single veneer or crown to the surrounding teeth can be very tricky. To properly do this, your dentist can send you to the lab that he uses for custom shading. Another solution is for your dentist to take color corrected and polarized photos with shade tabs for custom shading when the crown or veneer is redone.
Is the gap between your teeth in the morning? If so there is an easy fix. Your dentist can bond in a "fixed lingual retainer." that will solve your issue.
This could indicate that your temporaries are leaking, or your temporaries are picking up stain from something such as coffee. Likely your teeth are fine underneath. Make sure you contact your dentist so that he can evaluate and address your concerns.
