Let me add my welcome Aksana Sejanova, greetings from the forum and I hope the special day goes really well for all concerned.

I would agree a box is a good idea, then for example get your hotel on standby to steam press the dress to get it into tip top condition, or to engage a local agency or shop to assist. I don't think you need to get a second seat, but it is an option and I suppose has other advantages. You can only book the second seat via calling up however, you can't just make a reservation for 2 people - plus it should cost less, you shouldn't have to pay taxes on the second seat.