thanks much for sharing the solution. you are a class act. i wish more people who ask questions but don't receive a resolution from this site would post how they resolved the problem, even if they gave up.
i spent a short amount of time seeing if there was a documented statement as to what can be put in a cell. didn't find it in ibooks doc. i looked at keynote doc which had a list which i hope means that it would be an exhaustive list. i also hope it would be standardized across all products from apple. perhaps if i looked longer i could have found something more definitive. someone working a lot with table cells might have known this.
still, i think there is a sound argument that there is a bug here that should be fixed. if they will not be exported into the final book, they should not be dragged/dropped-able into a cell. perhaps there is a good reason why such an edit check is not performed.