Most seats don't come with extra packaging around the carseat at all in the box, some are even shipped in thick plastic carry bags. Some are lucky enough to have a plastic bag around the seat in the box, most do not even have that. The seat itself is exceptionally sturdy, much tougher than the shipping box, for sure, lol.
Yes, there have been studies to make sure seats still perform well after crashes, this one comes to mind and is the basis for NHTSA's car seat reuse policy http://www.nhtsa.dot.gov/people/injury/childps/ChildRestraints/ReUse/RestraintReUse.htm
(Though I will add that you MUST adhere to the carseat's manual directly, and possibly state rules that may mandate replacement by insurance if the seat is in a crash). Carseat makers also do extensive internal testing (Britax used to exchange crashed seats, their testing facility was full of crashed and re-tested seats when I visited once... this may have something to do with their following the liberal NHTSA reuse policy, compared to, say, Graco/Dorel who insists on replacement after any crash at all...maybe they have not retested their seats? Or their seats DO fail once crashed once? We can only speculate there).
Some seats only have the Date Of Manufacture on them, and then a description of expiration in the manual, though some have expiration dates stamped on the shell, and more now seem to have expiration dates on stickers next to the dates of manufacture. Plastic breakdown was not the reason they invented expiration dates, it really had more to do with ever-changing car safety features (primarily rearfacing babies being killed by airbags in front seats and the recognition that having seats without safety labeling expire would be a good way to keep future children safe). Though I'm sure plastic breakdown of course does have some validity in seats expiring and I would not recommend using seats beyond their expiration dates (which are trending longer and longer now, 9 years for the Britax Frontier, 8 years for Cosco and Sunshine Kids seats... 6 years or 'till December of the sixth year remains common, however).
Hope that helps! And the 'here' link in my siggy leads to the carseat.org technical encyclopedia that may have more answers for you