I once asked R’ Schachter how a Chuppas Niddah can work Bizman Hazeh. Because we do the Badeken first, we would consider her a Safek N’suah. But putting the ring on her finger would then be Assur because of Harchakas Niddah. He replied that Nisuin cannot be accomplished before Kiddushinin. If you didn’t start the process, you can’t finish it. However, if you did a Ma’aseh Nisuin before the Kiddushin then as soon as you do the Ma’aseh Kiddushin both the Kiddushin and Nisuin are Chal Miyad.
557: I have heard that R’ Rosensweig (the main Lamdan of the YU Roshei Yeshivah), combining the Shittah in Tos’fos and the Rambam (I think) says that you need Eidus L’kiyum Hadavar for the Badeken because it is an Eidus on a Davar Sheb’erva (you are changing her status from a P’nuyah to a N’suah; you could contrast this with what I said from R’ Schachter above but I think the resolution is fairly obvious). And since we are Choshesh (at least Lechatchilah) for the opinions in the Rishonim that any Passul witnesses seeing something Passel the whole Eidus on it, he therefore makes the Chassan be Meyached Eidim for the Badeken.