I an not able to reproduce the first problem. If i click on the Leistungen -> Nachhilfe I am brought to:
nachhilfe.php
clicking on the Leistungen link on that page brings me to:
leistungen.php
this seams correct to me.
to edit the link text for the menu items, edit the CSSMenuWriter/cssmw_content/menu.php and CSSMenuWriter/cssmw_home/menu.php files
all of the available documentation for Site Assist Pro is aon the Iste Assist Pro Support page:
siteassist/