The oldest paper I ever used (in 1969) was a box of Kodak double-weight glossy grade 4 made in 1942 and stored in a cupboard at room temperature. It was fine and only a little down on contrast. On the other hand, I have tried to use VC paper, particularly Kodak Polymax but also Agfa, which was 6 or 8 years old and found the low-contrast (yellow-filter-sensitive) emulsion had died and the paper was very slow and would only make high-contrast prints. Ilford Multigrade has been much better in this respect. Short answers to your questions:
1) A surprisingly long time (but see above).
2) Depends very much on type, but when paper does go bad, you will see veiled highlights, poor maximum black and even patchiness in the blacks. Many amateur photographers in the UK just after WWII (including me) began by using military surplus paper, which never seemed to give good contrast and which was so prone to chemical fog that Johnson's of Hendon made a special developer additive ("142" restrainer) to improve the paper's image quality!
Regards,
David