Never meld piety, ever... your skills alone are enough to keep MP stable. If such isn't sufficient just use piety food for progression.

Typical meld order goes Crit > SpS until desired speed > DH. Don't bother with det as the current healer kit is quite excessive and you shouldn't need the marginal extra healing compared to the more damage DH would give you over Det... there is also the issue that you will not always be able to meld Det, unlike DH.