Borderlands – How long until enemies respawn in Borderlands 2

Not only do most bosses respawn, even the final boss respawns.

However, the boss listed in the "Midgame spoiler" section below is replaced by a similar, new boss. And the second-to-last boss is the only other boss that doesn't respawn. Every other boss respawns 100% of the time.

Midgame spoiler:

Bloodwing does not respawn, but Son of Mothrakk spawns in her place, the fight is similar but he doesn't change elements and is generally much easier. Presumably he drops similar items.

Final battle spoiler:

Handsome Jack does not respawn, but the Warrior does. Seems you have one chance per playthrough to get Jack's Mask for this reason. You also won't get the generic loot that Jack would have dropped if you fight Warrior again, but Warrior drops plenty on his own If you get two Jack's Masks, keep one for a friend.

Any boss that drops unique loot respawns — except for the final battle (see the spoiler part above), but that boss can only possibly drop one unique item anyway: a specific skin customization.

That lever is used after finding all the ECHOs. It won't work because you're still missing one.

There are various videos of this mission floating around the internet. Here's one from YouTube.

Based on the video and some discussion in comments, it appears that the exact locations of the ECHOs are randomized among the containers in the room. But, they're just in lockers and boxes, not any special scripted locations. So, if you've opened all the other lockers and boxes in the room but one that won't open properly, it seems clear that the sixth echo is in that locker.

This is a glitch, and it sounds like it's one you've already seen in your game. Since BL2 doesn't allow manual saves, the only things that you can do that might work would be leaving the area, which you say you've already tried, or quitting the game and reloading it. If neither of these work, you're probably out of luck.