Just to clear things up a little:
1: Sanctuary does not stack in any way. Only 1 can be active on each tile, there's no way to stack heals with sanctuary.
2: 700k is not that much, it can be drained within seconds with 1 or 2 breakers. So I personally don't see anything odd here. ( I find the 1 hit story rather hard to believe )
3: Chainbreak, your calculations are off by a lot. Bacsojin cards are customized on our server to increase healing done by a lot more. There's no 100% cap on additional damage/healing to skills, there's only a % cap to race amplifiers.