Unmend and Unleash use Spell Speed for their GCD usage, case in point:

Current gear (565 Skill Speed, 354 Spell Speed)
Hard Slash GCD: 2.42s
Unmend/Unleash GCD: 2.50s

With a small bump in Spell Speed (565 Skill Speed, 395 Spell Speed)
Hard Slash GCD: 2.42s
Unmend/Unleash GCD: 2.48s

And with nothing but Anima Weapon so base Skill/Spell Speeds (354 Skill and Spell Speeds)
Hard Slash GCD: 2.50s
Unmend/Unleash GCD: 2.50s

Results: Pretty sure Unleash and Unmend use Spell Speed

Also, all MP returns from DRK abilities are based on your level, not your MP pool so Piety is a big no-go. However ALL DRK damage is based off your attack power plus the relevant secondaries regardless of whether the ability in question deals physical or magic damage.