There actualy is a way to save Ruben. You have to take a potion of healing in the 1st episode from when you throw it at the wither. Just don't throw it. Keep it in your inventory the whole time until Ruben comes out then it will give you the option to revive him.

What happens if you save Gabriel instead of Petra?

Just before entering the portal, Gabriel and Petra are about to get sucked into the beast. Save Gabriel - Picking this option will bring Gabriel down to the ground. Gabriel will appear in Episode 2 again. Save Petra - With this option, Petra will drop to the ground, but you'll soon be hit into the portal.

Can you kill Lucas in Minecraft story mode?

Lukas "dies" during Extreme Spleef. When Jesse chooses to save either Ivor or Petra, Lukas saves the one not chosen. However, Nell appears and digs the snow out from under Lukas and Ivor/Petra with a shovel.