Shortcut for cutting Pre-comps in Timeline?

Alt/Option + ] sets a layer out-point at the start of the current time indicator's position. This is different than cutting a video editing app like Premiere Pro, Davinci, Avid, or Final Cut Pro because you are at the start of the last frame. It makes sense in an effects app because you want to set keyframes where you can see the ending position or effect on the layer. If you want to move to the last frame, press Ctrl/Cmnd + Right Arrow.

When you create a Pre-comp, you can trim the new comp to the length of the selected layers. That is the standard workflow efficient workflow.

There is no automatic way to detect when another layer starts so that the layer below or above is trimmed to that out point. Your only option to find the first or last frame on a layer is to press "i" or "o" with the layer selected. "I" takes you to the start of the first frame of a selected layer, and "o" takes you to the start of the last frame of that layer. If you have two or more layers selected, the "i" and "o" keys will take you to the cuts, and then you have to use the Ctrl/Cmnd + left or right arrow keys to move to the right frame to make the cut and avoid overlapping frames.

No, there isn't. You're assuming intelligence/ smartness where there is none. I'm sure there's a script for this, though, so check AEScripts.com.

Mylenium