That is an excellent question. I've never tried re-wetting model magic after it's fully dry, but considering how fragile it is, I'm not surprised it would fall apart. I know Akuriko (akuriko.deviantart.com/) created her armor for Zelda out of model magic and then covered it with resin. While I haven't tried it myself, it could be a great way to reinforce it and seal it from moisture.
For your last photo, I would love to see how you make the shape of the armor close fitting like that, I am working on my cosplay of Vayne from league and this is just a wonderful tutorial! I am doing her aristocrat skin.