1) Probably about what you would expect to result from this situation: something bad
(yes, this is to scale, or at least as best I can manage with a measuring tool)
Phobos would quickly cease to be an asteroid, and Chomolungma would quickly cease to be Mother of Mountains. In a spectacular cascade of collapsing rock, the landscape would be changed forever -- mountains and valleys scoured clean in an expanding disk of rubble behaving more like a fluid than a solid. The energy released in this collapse is on the scale of 200,000 megatons of TNT, or about 10 million Hiroshima bombs.
Moving with it will be a huge displacement of the atmosphere. A similar effect happens with large pyroclastic flows, where a violent wind capable of shattering trees surges well in front of the main mass of volcanic ash and rock. Except in this case, it's not a piece of the mountain rushing downhill that's causing it -- it's the entire mountain plus a minor moon.
How far this devastation would reach is hard to say, but it should suffice to conclude that I would not want to be among those mountains on that day.
2) Same deal, just replace the Himalaya with the landscape around the South Pole.
3) A very big splash, lots of rubble among the trench. I would not want to be on or near the Pacific Ocean on that day.
4) Meteor crater is in Arizona, but either way, I'll let the image do the talking: