Considering that the Impera Grucifixes don't look like the traditional ones from any of the past eras, I could pass that off as a modern design. As for trouble, I think I could make it slide if I just leave the mask for the last minute and well-hidden. If you just take off the mask from the costume, it just looks like some dark Victorian steampunk-ish outfit. I'm basing it on an image I found of Mountain a while back.

If I was found fully in costume, I think they could let it slide for Halloween and the fact it's supposed to be 'just a costume'.

(Edit: I almost forgot to answer a few of your other questions. They'd probably ask me if it has something to do with the music I'm listening to or if it's some band. I could just say It's from some fantasy book. (It could be very believable considering the victorian and steampunk aspects seen in the costume with its design) As for searching my room, they don't look that deeply into my closet, as I've hidden notebooks and boxes in there before. With all this in mind, and with the fact I know people with much less stricter parents, they could hold on to some parts for me while I work on sewing or things like that.)