thank you very much for the information! It's quite interesting, I didn't know that webkit is as restrictive as you are telling me. Very valuable information for further work. Currently I'm not able to continue testing, but will have a close look at the things as soon as possible.
You're right, mostly I'm trying to guarantee that there are always a couple of lines after headlines, but I'm also dealing with images and corresponding captions for which I already used the page-break-inside:avoid; property for an outer div. But it didn't seem to have any effect in iBooks. Unfortunately ... Maybe I have done something wrong way, but I'm afraid that iBooks is just kind of hardheaded in that context. I'll give it one more try anyway.
So, many thanks to you, have a nice weekend!