As I could not get any statement on some topics here yet, I tried over the weekend to find some workarounds. Maybe those workarounds are interesting for others too, so I post them here:
This one is about a bug I found in the menu structure of at least 3 CK templates I tried (Corporate2, bikeshop and postnote).
The effect was, that the main menu did not work for child entries and those pages just showed a kind of copy of the parent page combined with a 404 message (File not found), while they worked fine as soon as I moved them up to top level.
Source of this was, if the parent and child page both are based on "single article". As soon as I changed the parent page to any other menu item, then the child page worked fine again.
So, changing the parent to e.g. category blog (which only has one article then) solved that issue - until Gavick will fix this bug.
H.