This is info I gathered from the Sacramento Bee.
The auxiliary spillway which is accessed at a higher level than the main concrete one in the video is the one to be avoided at all costs. That one is the one that was causing dangerous erosion of the hillside back to the dam.
Since the evacuation and prior to the latest storm, the water level in the lake has been lowered almost 50 feet.
Last I read, engineers believe that by lowering the lake 50 feet and by using the main spillway at the greatest capacity it can be used safely, the level will not rise to the level of the auxiliary spillway.
The sideways overspill you see on the main spillway is the result of the huge crack/sinkhole that was there and which they attempted to shore up. One would assume they knew that what’s happening would happen and are okay with it.
The water flow through the main spillway can be controlled, but if they limit it too much, the level may rise to the auxiliary which they want to avoid at all costs.
In any case, if I lived downstream, I’d be outta there.
That is why it overflowed the emergency spillway in the first place. They wanted to limit the damage to the main spillway and they throttled it back. Now they have no choice, one would think. Of course, they said it wouldn't overflow the emergency spillway 2 days before it. Lord only knows what they will do.