Here are my questions:
What kind of rock did you add to the pond and how much (was it true live rock, or dry rock). If the tank is 360-400 gallons, you need a bare minimum of 200lbs of rock (I would recommend closer to 1lb per gallon or 0.45 kilos per gallon). Real live rock is vastly superior to dry rock, especially if you want to cycle a tank quickly.
Have you been testing the water and tracking how much ammonia the pond has been able to process? Can you test ammonia, nitrite, and nitrate daily to gauge whether or not the tank is processing ammo
Do you have lots of extra saltwater on hand so that you can do big water changes in the event of an ammonia spike?
Do you have or can you get some Prime to neutralize ammonia should you get a spike that isn’t controlled by water changes?
And lastly, why won’t the retailer let you postpone? Have you explained that the tank is not ready and that they will potentially die?