They don't always cast in the same place, its just the randomness of AoE spells. I find I can get a decent sized group of troops blessed with a couple holy-2 priests casting multiple blessings. Sometimes a few troops on the corners get missed. I don't usually need to do this but you could try having the priests slightly seperated from each other?
Yes, Hold&Attack is forced. You just can't try to rush the enemy and except your troops to receive their blessing.
Also, IIRC Pangaea has 1N3H dryads in base, and 1N2H dryads in New Era. That would mean base Pangaea is a viable bless race, espcially as White Centaurs are quite good sacred units.