I've been watching this thread to see if someone came up with a solution, because it could have some bearing on users of ckgedit and fckg. And I see this morning that you've posted to the German forum.
I have no solution, but I can confirm that your experience is correct: the data entry form does not display in the included page while it does display when the data entry page is accessed separately. I believe I see the reason for this. The data entry form is attached to
a particular page. When the data entry form is on page_1 and included in page_2, it does not display on page_2. But when you access page_2 separately, there it is.