Q1: Am I able to access the output filename (without extention) inside the @startuml?
Q2: Why this does result in the full file name?
Q1: With last beta https://www.dropbox.com/s/koo42q3d9gxw288/plantuml.jar?dl=0
you can have:
note right: %filename%
note left: %filenameNoExtension%
Q2: Not sure to understand the question. This is simply our decision (see http://plantuml.com/preprocessing )
(Note that this is not working with the online server http://www.plantuml.com/plantuml because there are no "filename" in that case)