1. You cannot have two layers at 100% opacity as the top layer will completely block out the lower track. Anything less than 100% opacity will let varying degrees of the lower track show through. BUT what it sounds like you wanyt is a crossfade transition. To do this you place both events on the same track and overlap one over the other partially. You will then see a diagonal line (more or less) in that overlapped area representing the changes in opacity. You will change from 100% opacity of the first clip to 100% opacity of the second clip through a gradation process. First clip will get less opaque as second clip gets more opaque. This is a cross fade or a dissolve transition.
2. To reverse a clip insert a velocity envelope in the clip (see ‘insert-envelope-velocity) You can take the velocity of the clip to 0 (or a still frame) or even reverse it to play it backwards by right clicking the line of the envelope and placing nodes and then dragging the nodes up and down.