You will have no problems with oxygen with the pumps off.

I would guess the waterfalls have a leak behind them, when the water volume is high, water will back up the stones. Any small leak will amount to quite a lot of water. I highly doubt 1/2 that volume of water is evaporation per day unless the water temp was well over 90F.

This is one of the reasons I don't have a waterfall in my garden pond, I am on a well and cannot keep topping things off due to leaks. Leaks are inevitable with running water features.