increased spelldamage mechanics on wands

"10-20 physical damage" and "10% increased physical damage" are different things - one is the base damage, one increases that damage.

"10% increased spell damage" and "8% increased spell damage" are not different things. Neither of these things itself is an amount of spell damage, so neither of them can apply to the other. Both of them are an increase, so they will stack and will both apply to whatever the base damage of your spell is when you use it.