This post [1] from that thread is the simplest presentation of the current and expected scenarios than anything I've seen elsewhere, including the OP. If they are able to run the main spillway at the current rate of 100,000 cfs without concern for further major erosion, the outflow outpaces anything but the highest inflows from the recent storms.

[1] https://www.metabunk.org/oroville-dam-spillway-failure.t8381...

Those are the most informative pictures of the situation I've seen yet.

Is welding giant metal sheets over the hole an option?

No. You would need to anchor the metal plates to something, which means you'd need to drive spikes down into bedrock and then weld things to the spikes.