Why not use only one support vane for the secondary mirror, to avoid multiple diffraction spikes? Each vane holding the secondary mirror in front of the primary causes a diffraction spike. Couldn't the secondary mirror be held with only one vane?

If stability is the issue, couldn't it be substituted be steering the vane to hold the secondary in place with precision, similar to active optics, regardless of winds? And why does the James Webb Space Telescope use three vanes when it will operate in undisturbed microgravity and shadow?

(Maybe the star spikes are desired 'cause they make stars look nice?)
I can't speak for the JWST, but I do work at a company that designs telescopes for spacecraft and I can tell you that a lot of the structure of the telescope is to protect the instruments during the initial ascent to space where the launch vibrations and g-loading will be the worst. Plus mirrors for telescopes that big have really precise alignment tolerances and if the secondary mirror isn't very rigidly attached to the spacecraft even simple microstrains from heat loads or spacecraft pointing can move the mirror out of alignment. Even the fact that the telescope is going from a 1g environment to a 0g environment can move the mirror because it was aligned on Earth when the beam was slightly bent and then it goes back to being straight in microgravity and the mirror is misaligned! It stinks that there is stuff in the way of some beautiful science that you're trying to take a picture of, but it's the only way to make sure the telescope survives up there!