I have solid maple boards that were originally intended to be cabinet filler pieces. The boards are 3/4 x 6" x 30&42 made of two 3" glued pieces. They are finished on all but the back side.

(A large cabinet company closed shop and by the end of the closeout, they knew me so well they just gave me three pallets full)

Questions:

1) Can I use this for flooring?

2) Would I need to remove the finish, or could I put the finished side down?

3) Would I need to make grooves on the bottom side?

4) I have some adhesive backed underlayment (Instalay) that could make this really easy...or maybe easy to undo

5) Other things to consider?

Yes you can use these pieces of maple for flooring. It does not need to have a tongue and groove. You could glue it down in a random pattern over a suitable sub-floor. The raw wood side would be glued down with a wood floor adhesive.