Prologue

I didn't play through the Prologue, is it the first part of the game anyway?

Andi.

As a newbie I'm very happy about the Prologue-Campaign - it's a fine tutorial...

I only checked out the prologue after getting stuck in the campaign. Was surprised to find it there. Why doesn't the game start with the prologue then and asks you, if you want to skip it?

I think it makes sesne, the prologue is basically an "un-tutorial" making it substantially harder to get into the game than starting with the first mission of the game proper. If you haven't learned all the controls prior to the prologue it would be a wretched experience indeed.