To answer your questions:
1. Spells do not stack, you can only use 1 per Attack phase at a time.
From the rules:
You may now use any one Spell you have in your possession. Once a spell is used, it is consumed and discarded. Set its token aside.
2.Poison lasts until battle is complete.
In Combat, for the remainder of the Combat sequence, inflict 5 extra damage per turn.
Hope you're having fun!