Speaking personally I don't think I'd be interested in working on any musical project unpaid for an indefinite amount of time unless I had some kind of ownership - like if it was my own project, or a band I was a member of. If you were my friend I'd be more than happy to turn up for a few hours in a studio and sing what you told me to, but committing indefinitely to working on someone else's shit? Where's the upside? Maybe the singer just didn't like your tunes enough?
Looking for a silver lining here: at least the dope of a singer bailed now before you were a lot further down the line with the project and had put a lot more work in. Maybe they bottled it because of the celebrity interest, maybe not. I'd rather have someone quit on a project sooner rather than later. Someone as unpredictable as this is very unlikely to be any kind of joy to work with, and very likely to baulk at the kind of schedules and work that being in a successful artist involves. It totally sucks, but at least you weren't dragged on further before they did this.