How can currents and electromagnets encounter frictional effects?

When you slide a strong magnet quickly across a metal surface, there is a friction-like magnetic drag effect. This effect occurs even when the two objects don't touch. The origin of this effect lies in the repulsions between the metal and magnet: it's strongest slightly in front of the moving magnet so the magnet encounters some difficulty heading forward. The reason why the magnetization of the metal is strongest slightly in front of the moving magnet is related to the loss of energy by current moving in the metal. The magnetization (of the metal surface) in front of the moving magnet is fresher than the magnetization behind it. The current responsible for the magnetization behind the magnet has been flowing for long enough to have lost energy. But the faster you move the magnet across the metal surface, the less time the currents in it have to lose energy and the less friction-like force the magnet experiences.

Without electric current, a magnetic field changes over time to cause an electric field. I don't understand this concept.

I'm not sure what this question refers to so I'll guess. If there is current flowing steadily through the wire coil of an electromagnet, that wire coil and electromagnet will be magnetic. It will have a magnetic field around it. Since that magnetic field is steady (not changing), it will not create any electric field. But if the current in the coil is suddenly removed, the magnetic field around the electromagnet will begin to disappear. This changing magnetic field will produce an electric field around the electromagnet.

If you spin the (floating) magnetic top in a vacuum, would it spin (theoretically) forever?

Probably not. The top and the base are exerting forces on one another all the time, by way of magnetism. If those forces are not perfectly balanced around the top's center of mass, they will exert a torque on the top. Most of the time, such torques will slow the top down and it will eventually spin too slowly to be stable. It will then flip over because it's in an unstable equilibrium.