Why are there 2 bars when looking at a weapon’s stats

Essentially you lose everything. It's like starting over again from level 1.

This time around you can pay Prestige Tokens to get early unlocks, (ie, the AK-47 or Steady Aim at level 1) but the weapon/perk/gear's status is reset and you will have to level it as you normally would on each Prestige. For instance, if you had gold camo on a gun and all the attachments, even if you use a prestige token to "Unlock Gear," you'll lose the attachments/camo and you'll have to level the weapon again.

The only thing that is saved from one prestige to the next is any Titles and Emblems you've received from doing challenges. The challenges themselves reset, however. If you're really close to the end of a challenge that unlocks a title or emblem, it might make sense to finish it before using your Prestige.

You also get to keep the quantity of custom classes, although your custom classes reset to the defaults when you Prestige. You'll also not have access to custom classes until you reach the level where they unlock on the first pass.

The order of the perks granted by Specialist is the order they are selected in. For example, I can set it up so that Assassin is my first extra perk, Extreme Conditioning is my second, and Stalker is my third.

The only restrictions placed on what perks may be selected are the three perks you already have selected outside of Specialist, and perks that you have not yet unlocked.

Just deselect all of them, and then select them in the order you wish for them to be applied when getting kill streaks.