Personally I agree that to Tuon/Fortuan and probablly the Seanchan nobility swearing to the Crystal Thone is just another way of saying he must swear to the empress. It would be a bit like oneone giving their oath in our world to the crown. It was not an oath directed to the band of metal worn on the head rather it was meant to be an oath to the office of king/queen.

I do think that if its possible the Seanchan would prefer to have Rand actually swear to the empress while she is sitting on the actualy crystal throne simply because the ter'angreal would make it more likely that he would keep any oath he made. If they could secure Rand etc and use him to reclaim the ter'angreal throne I am sure they would at least consider the idea.