Are datasheet min/max values "guaranteed" for the lifetime of the part?
The datasheet describes what the manufacturer has to provide to you in a legally binding fashion. So, unless it is noted otherwise in the datasheet, everything which is in it has to be correct under the conditions specified in the datasheet.
You have used the term lifetime of a part. It is also often not well defined, if defined at all. It is the designers responsibility to understand how the parts work, and which part is the most critical for the products lifetime, and do everything reasonable in his disposal to protect that part.
Of course it is virtually impossible to reliably determine whether the part was operated within the specified conditions, which makes this all promise thing a little vaguely controllable. There are some failure processes which are exponential function of the temperature and stress, which property can give you some confidence if your product's operating conditions are less challenging.