Borderlands – I’ve restarted the campaign by accident, can I cancel this

You need to go to Select Character from the main menu (you can't be in a game when you do this), then select the character you want to play Playthrough 1 with (even if they're already "selected"). When you click them it will give you the option for "True Vault Hunter Mode" (playthrough 2/2.5) or Playthrough 1 ("Normal Mode", AFAIK). Just pick the first playthrough from that menu and hit Continue, you'll find yourself back in Playthrough 1 exactly how it ended, all of your old missions intact and enemies at level 30 max.

This is different from how Borderlands 1 worked for this, but it takes away one extra menu every time you try to play the game so that's probably why it was moved.

In playthrough 1, outside of the DLC, there's no such thing as scaling. Enemy levels are determined solely by the area they're in and your mission (certain missions effectively "level up" areas, notably Sanctuary and The Dust). You can easily check the level of an area by checking out the vending machines in the area, they sell items scaled to the level.

DLC in playthrough 1 scales to your level...ish. It maxes out at level 30. I played the DLC as a level 34 character and was disappointed to see everything was at level 30 in playthrough 1. Not sure if it has a minimum level it scales to.

Playthrough 2 works the same way until you beat the Final Boss; then everything in that playthrough scales to your level (except mission rewards for missions picked up before the final boss).

Playthrough 3 works the same was as Playthrough 2 after the final boss: Everything scales to your level, period. In team play, it will always be scaled to the highest level player's level. And you have to be at least level 50 to unlock it, so enemies are effectively level 50-61 at all times. Though if you take a mission before leveling up, the mission will still stay at that lower level, as always.

Basically the game works like a standard RPG without level scaling until you A) beat the game twice (because you should be maxed out anyway) or B) enter a DLC (because you can do that at any time so a preset level would be a really bad idea)