That's not true. Even a non-magnetic metallic object conducts electricity, and an electrical conductor experiences a force when it moves through a magnetic field.
If you take a piece of copper and push it between the pole pieces of a high-power magnet, you will feel a resistive force, as though you were pushing against an invisible viscous medium; this effect is described by Lenz's law.
Also, there is an all-plastic trombone called a "pBone," made by Warwick Music Ltd. in the UK.
Yes. My son and I march in the Ohio State University alumni marching band every year and several alumni members started using the plastic from ones. For one thing they are much lighter and play with pretty much the same quality.