Not 1. It's the wrong product as its basically a filler, and hasn't got the strength to hold the fibres together. And you'd actually sand first, use the product, then sand again.

Not 2. You aren't addressing the basic problem of lack of structural integrity of the fingerboard fibres. Sanding improves the feel, and will be necessary anyway, and lemon oil improves the looks (for a while) but again does nothing to hold the bits together.

3 is a possibility, but whilst the finish will help hold the board together, it won't do as good a job as using thin CA.