It'd be highly susceptible to erosion, and the channel wouldn't be stable. It'd either expand gradually over time with all sorts of ecological ramifications, or deposition would clog the channel constantly. Who knows, might do both depending on seasonal factors? Imagine the damage a hurricane might do to those massive cliffs, for example.
That all sounds quite different from an immediate disaster. Those things apply to any river and those typically have a slope well in excess of that. Not that I would argue in favor of cutting a channel between the two but it doesn't sound as though this is an insurmountable problem, basically well within the elevation difference that even a single minor lock would be able to handle.
You might even have to add a pumping system to the lock to ensure that it would fill up fast enough.
Tides would take care of the filling. The Pacific tidal range is something like +-6.5m, the Atlantic side is <1m, plus whatever offset due to weather and phase error in the tidal times due to the earth being a sphere. A system of locks could handle it, that's exactly what the existing canal is after all. It's just more expensive and impractical to construct/maintain the deeper into the continent that canal cuts.
Ecologically, it's mainly about connecting the Atlantic and Pacific hydrologically. They're different temperatures, salinities, turbidities, etc.
Yes, but whatever the elevation difference if it were done at sea level (presumably the average between the two nominal levels on both sides) there would always be less water to be displaced in the resulting locks and that would in itself be a win, but the amount of material displaced to make the channel may well outweigh that by a considerable margin. I take it the engineers have run the numbers and decided that the current solution is the optimum or very close to it in terms of economics.
