1. Does Widespread Ruin target?

2. If it doesn't, if my opponent has Dark Magician Girl and Dark Magician on the field, and DM declares an attack and i activate Widespread, if my opponent were to chain with Book of Moon or Compulsoary evacuation device on DM, or Magicial Dimension on DM to summon a new spellcaster (let's say its weaker than DMG), will the DMG be destroyed instead?

no, dont target

yes it would kill DMG

Cards that do not allow you to select the target at activation will not care what monsters are available when it attempts to resolves, as long as in this case, the one with the "highest ATK" is destroyed.

So, if the highest ATK monster ends up being a Sheep Token after your only other monster is flipped face-down, the Sheep Token would still be destroyed.