I just spoke to both Allianz and ABD. I got somewhat conflicting stories, but I think in the end it's all good. From the very nice Allianz rep - there are no restrictions against changing travel dates or adding additional coverage. I'm sure I don't have all the details exactly right, but she said ABD doesn't send the policyholder information right away. They hold it and send it in a batch (I don't think that is the word she used, but it conveys the meaning, I think

) So Allianz can't modify the coverage. She said I would need to call Disney and have them modify it. Of course, ABD had already told me to call Allianz. So I called ABD again and spoke to a very pleasant and helpful vacationista, who told me that Allianz
can change dates and add additional coverage. She said they have run into this issue before and if I needed to add more coverage and the Allianz rep claimed they couldn't do it, to call ABD and they have contacts at Allianz who can take care of it.
And here's the story from ABD on which flights are covered. If you have multiple flights under the same confirmation number they are covered. She also told me that the segment leaving the US is covered regardless of whether the final destination is the origin of the ABD. So in my case I am flying on Delta from Nashville to Edinburgh and from Shannon to Nashville and those flights are covered. I booked a separate flight with Aer Lingus (different confirmation number, obviously) from Edinburgh to Dublin (where the tour starts) and that isn't covered, which is okay with me because it was really cheap.
Clear as mud, right?