Divine Seal should not be treated like a DPS class' cool down (must fit as many high damaging skills as we can in this duration!! Or in your case, fit as many GCDs as we can!!).

IMHO the main use of Divine Seal is :
Getting through a particular phase in the encounter with the least amount of mana used.

Additionally your big pack pull rotation, I would not recommend the usage of Swift Cast on Medica 2.
IMHO, Swift Cast is best used for your first Holy for the following reasons:
1) Able to PoM during the 2.5 sec waiting time after 1st Holy
2) The initial stun can go off 3 seconds earlier.
3) The 2nd Holy's stun will guarantee be applied (Explanation : If you're above a certain Spell Speed, if you chain cast Holy, when your second Holy's damage is registered, the mobs are still stunned from your 1st holy (4 second), thus your second holy will have a "Stun : No Effect", and you gotta wait til the 3rd Holy for the 2nd 2 second stun).