Maintenance for the week of January 20:
• PC/Mac: No maintenance – January 20
• NA megaservers for maintenance – January 22, 4:00AM EST (9:00 UTC) - 9:00AM EST (14:00 UTC)
• EU megaservers for maintenance – January 22, 9:00 UTC (4:00AM EST) - 14:00 UTC (9:00AM EST)
Imperial Edition is in the crown store, or was, last I checked. Look under upgrades.
The Explorer Pack no longer works, last I heard. It doesn't expire on the card, but the promotion has ended, and the codes are no longer honored (or something, I'm a little unclear on this point).
The only mount code I know still works is Elder Scrolls Legends. If you buy the Dark Brotherhood DLC (for that game, not ESO's Dark Brotherhood DLC), you're supposed to get a PC/Mac code for a Doom Wolf. (Haven't tested this personally, so I'm unsure.)
As far as I know, the Dragon Warrior costume codes included with Skyrim's PS4 and XBone releases do still work. The catch is, they're locked to that platform's servers. So, a PS4 copy will include a code you can use on PSN to get the costume and mount. The XBone copy will give you a code for XBL. But, you can't take a code from one of those and apply it to the PC server. I don't know why that is. But, I haven't been rebuying Skyrim for every platform.