The most efficient way to learn/cast Shadow spells at a lower level would be with scrolls. This won't let you cast a Shadow spell as an X-1 level spell to emulate an X-1 level spell effect, however it will let your villain learn/cast theses spells at a level lower than what he would be able to normally.
A prepared caster class you can learn the spells by studying a scroll containing the spells. Adding the spell this way requires the character to decipher the magical writing then a DC 15 + spell's level versus your Spellcraft to add it to your spellbook. You still won't be able to prepare them until you have a slot you can cast them in, but they will have learned them.
To cast them at a lower level, you can use a scroll to cast the spell at ANY LEVEL. There are two main ways to cast from a scroll. To cast the easy way, the scroll must be the correct type (arcane or divine), must be on your class list, and you must have the correct ability score to cast it. If your Caster level is equal to the scroll's, then you automatically succeed. If not, you must succeed a DC = Scroll's caster level + 1 versus your caster level to cast from the scroll. If you don't meet those conditions you can still cast the scroll with a Use Magic Device check with a DC of 20 + Spell's caster level. For this you still must meet the ability score to cast it, but you can also emulate this with a separate Use Magic Device check.
As a bonus:
After adding the Shadow Spell to your spell list from the scroll, you can use the Scribe Scroll feat to create more copies of said spell, thus granting your villain an infinite supply of Shadow spells!