You should be able to raise the level cap without needing to invest any more skill points in Summon Demon; I learned it to level 15 without putting any skill points in.
On your enchantment platform, when selecting an enchantment to add to your sword, scroll down to any level 10 enchantment and check out the required ingredients - a malachite gem will be listed. If you don't have any level 10 weapon or armour enchantments yet, switch to enchanting a piece of jewelry and select any healing aura.
Level 9 of most enchantments do not require a malachite gem, but have almost as much of an effect as level 10, so I stuck to level 9. With healing auras they always require a malachite gem, so I only used level 10 (being lucky enough to find that formula after a bit of reloading in the Broken Valley enchanter's cellar).
