1. They are still velocities, and hence, keep m/s
2. No, it has only one horizontal and one vertical component in one plane, because if you take the smaller ones, you would have got another resultant velocity, which is not what was given. You can 'complete the parallelogram of forces' (or rather, of velocities)to see by yourself.
3. Have you studied how a force can be expressed into two or more components? Here, the same principle is used and to make it easier, the horizontal and vertical axes are chosen.