See #13.
On the one hand I agree that the dam (despite it looking “fragile” in the video I linked to) is not going to fail, assuming that “mountain” (or big hill) is composed of very solid material.
OTOH, from commenter Smith’s comments in your post, I take it that it IS possible to lose up to 30 ft. of emergency spillway “cap” (I suspect that’s the wrong term) plus up to 100 ft. of material below it. Call the resulting discharge 1 million acre-feet of water, plus whatever flows into the lake as the break progresses. We know peak inflow can be up to 200,000 cfs: if we go with 150,000 cfs for 4 days, that’s an additional 1.19 million acre-feet. So, what the heck, make it 2 million acre-feet headed downstream. That compares to 3,537,577 acre-feet for the entire lake. A failure of the emergency spillway is going to be a pretty big deal, mitigated a bit by it almost certainly not happening all at once.
I also question the “solid bedrock” business a bit. Do we KNOW how solid that rock is - even the undisturbed stuff? Someone posted here that a 2nd power plant was never built because the “bedrock” wherever that plant was going to go was too poor.
Finally, commenter Smith also refers to any flood from an emergency spillway failure as being “in line with historic floods”. I assume that does not include 1862...