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Nail polish remover is fine to use, it is basically acetone with fragrance. As far as solvents that will remove or soften paints go it is a safe-ish one. Other than it's flammability.

Yes plain...

If the glue line/break is pretty long (lots of surface area), I would use plain old white glue to glue the bone back together. White glue (PVA) works well with bone and ivory.

If that repair does...