I'm wondering about the best way to proceed through Blood 2.
Do I start with Nightmares, Caleb, Caleb under Chosen, or one of the other Chosen?

Nightmares is the expansion pack-probably want to to play Chosen first. If memory serves,Caleb gives you all the cutscenes in the game but the other folk just let you play without them.

When you click shortcut and the launch menu pops up, on the bottom left of it is a check which says "Nightmares Add-On".Uncheck that! Click "Launch Blood 2" and. Chose "singleplayer" when you get to the main menu of the game. Play "nightmare levels" after you complete Blood 2:the chosen. Anyways, have fun!

For regular Blood 2, choose "Caleb" and for Nightmares choose "Nightmares".
This will give you all the cutscenes.

While drunk.