Hi. I can only imagine how frustrating this must be and I'm sorry you're having trouble. I did some searching to see if I could help. While I found nothing definitive, in general it seems that syncing iOS devices and Yahoo with CalDAV is not a good idea.
There is some information in this thread that seems to have helped some people:
Please let me know if any of the ideas in that thread help. Meanwhile, I will keep searching and post here if I find something relevant.