Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero?
They have to be at very low temperatures, that takes a lot of energy.
The ATLAS Barrel Toroid was first cooled down over a six-week period in July-August to reach –269°C . It was then powered up step-by-step to higher and higher currents, reaching 21 thousand amps for the first time during the night of 9 November. This is 500 amps above the current needed to produce the nominal magnetic field. Afterwards, the current was switched off and the stored magnetic energy of 1.1 GigaJoules, the equivalent of about 10 000 cars travelling at 70km/h, has now been safely dissipated, raising the cold mass of the magnet to –218°C.
So it also takes a lot of energy to build up the magnetic field and the energy is stored ; one has to be careful in switching off that disaster does not happen.
Since resistance is zero, does that also mean that the time it takes for current to flow & for the magnetic field to induce is instantaneous?
There is the bound of the velocity of light, for everything. Nothing is instantaneous in electromagnetism of anything else.
The issue of self-inductance is irrelevant?
In charging the magnet and in a controlled quench it is relevant.