You can't wear the cross necklace, or have the bumper-sticker; they signal religion. And you can wear the halloween mask, or the welding mask; they don't signal religion. There's nothing more to it than that, and there's no complex interaction.
If you can still see the hijab under the welding mask (and you can; the welding mask doesn't cover the back of your head, or your neck) then the hijab has gotta go. (And, to be extra clear, it's a unilateral rule: nuns and priests can't wear their vestments in public either! They've gotta change into them at the church/nunnery, and change back before they leave.)
On the other hand, if you wear a cross necklace under your clothes where nobody can see it, that's perfectly fine. Because, again, the point is that you can't signal your religion. The symbols aren't against the law; displaying them is.
And so, actually, wearing a hijab would be fine if you wear, say, a motorcycle helmet over your head, and a bandana over the lower half of your face. You're covering all the same area—but, because nobody can tell that you're wearing a hijab, and because neither a motorcycle helmet nor a bandana signal religious affiliation, you're fine. The symbol is hidden, like the cross under your clothes is hidden.
(Interestingly, since the point of the hijab tradition is to cover these areas, not to look a specific way, anything that covers these areas is "functionally" still a hijab as far as Islamic social norms are concerned. So wearing just the motorcycle helmet + bandana would actually be a perfectly legitimate way to cohere to your religious beliefs, without [legibly] signalling your religion. ...until, that is, every Muslim woman started doing it; then it'd become a legible religious symbol, and so be banned as well.)
