Naming Logic

Not sure what you are trying to say. It does what it says on the tin - the start and end in relation to the path length and in your case after applying any other shape layer effects based on your stacking order. The only thing I could imagine is the lack of visual feedback on the path orientation and parameters liek actual length, but that's always been this way.

Mylenium

The Start and End values tell you where on the path the starting and ending values for the trim are.  If the Start value is at 25%, then 1/4 of the line will be hidden because it does not start until 25% of the path is covered. If the End value is set to zero, you won't see a path because the end value is at the start of the path. It's all based on the first vertex. 

The offset value slides the start and end values along the path. Set the start to 20% and the end to 30%, and you have a visible segment of the line that is 10% of the path length. Animate the Offset, and 10% of the line will travel along the path.

I hope that clears things up.