1)Does their STR bonus while Raging stack with other STR bonuses (potions, Bull's Strength spells, ect.)?

2)Does their speed bonus apply while wearing heavy armor (I can't find anywhere in the rulebook saying it doesn't, but I know in D&D 3e they should slow down to normal speed in heavy armor)?

1)no

thanks

3) When a rage ends, and your CON drops, can you die (or will the game leave you at 1 HP)?

3) the latter