Welcome to Photrio.
Second question first - no. There is just one package of wrapped/enveloped paper in a 100 sheet box. So the possibility exists that the paper inside doesn't match the label on the box. However, it also might just be a case of a user being extra careful - dividing the contents of the box between two or more envelopes, to help minimize damage if one such envelope is accidentally opened in the light.
As for your test, it is a bit problematic.
Film developers like DDX are definitely less active than paper developers. So it is possible that development in DDX won't reveal problems with fog that paper developers would reveal.
In addition, even if the paper passes a "fog" test, it is possible that its maximum density/black will be affected by age, and/or its contrast behavior will be affected by age.
That being said, even if there have been age related changes, the paper may still be capable of making good prints. If that is the case, it just means that if you try to reprint a negative on newer paper, you might have to adjust for the differences.
Paper like that is great for contact proof sheets