1. Not sure. I never bothered (I smoothed things out while shaping and then coated with several layers or primer to cover small pits), but you could try it on a scrap piece. Be sure to wear a dust mask, though.
2. I used acrylics for the pendant and swirly buttons on my Firefox cosplay.
3. Definitely worth trying. I don't know if (long term) the model magic would react with anything in the glue or not. I used gesso because I already had some. Sealing may not be necessary for all paints, but the metallic gold I used turned green on the pieces I didn't seal (something in the model magic was oxidizing the copper flakes in the paint).
4. I haven't had problems, but if the horns are really pointy/delicately branched, it might be an issue. (I haven't deliberately tried to damage them, but since I'm kind of clumsy, they've been thwacked against a variety of hard surfaces, stuffed under heavy things in luggage, stepped on with bare feet, dropped onto hard floors, and sat on... No breaks so far.).
- Avatar Kyoshi (Avatar the Last Airbender), Ashura (RG Veda)
- Princess Alexandria (Red River)
- blue and white hanfu (most of Shusui's blue and white outfit (.Story of Saiunkoku)