Originally I planned to grind a Balance wizard to level 100 in order to buy Harrowing Nightmare Packs on him in hopes of getting level 100 balance gear. This is no longer possible, as I hadn't been playing as much as needed in order to do this. To my knowledge you cannot buy packs and not open them, they'll just automatically open themselves. But, if I get a friend to gift me around 50 packs, can I wait until I'm level 100 and open them a few weeks after the Harrowing Nightmare Packs go away, and still get the level 100 items?
Yes, packs in your gift window will stay there and can be opened at a later time (and level) if you wish.
Just a word of caution though. Don't make the mistake I did and gift yourself a lot of packs for when your wizard levels up, or you'll basically overload the system and lose all of your packs. I had to open a support ticket.
I'd prefer if you could just level pack gear up though. Then there'd be no need to stockpile packs for later.