For 3 wrong above I would wager you have somehow selected all four verts (2 on top and 2 on the bottom) and then moved all four. To avoid this kind of confusion I would recommend using the "merge vertex tool". With it you will avoid having to merge the verts at the end and you get positive feed abck as each vert is merged.

Don't fret about making mistakes it is how you learn the quirks of the application. Learning what causes the kind of behavior you were seeing is a valuable thing. I am playing with several modeling packages and even though I have modeled stuff for a long time I still get wrapped around the axle especially going between packages.

The key is to learn from this and keep pressing forward. I guarantee this will not be the first "scratch your head for hours" Maya moment. We all have em. The options are figure it and learn, do it another way and don't worry about it, or give up. If people picked option three, there would not be a Maya modeler on the planet (lol).