Any casting or machined metal in the Tesla would have to be flawless internally.

Only rotor. And that is not that hard.

At 18K, ball bearings would need forced lubrication, as the centrifugal force would strip them of lube very soon

Or you should use sealed ball bearings. Also, centrifugal force for bearing much less that for the rotor, since diameter of rotating parts is much smaller. Vaccum cleaner turbines runs at 20-30K RPM, and ball bearings perfecly OK without forced lubrication for many years. SKF gives limiting speed for randomly taken W628/8-2Z bearing at 45K RPM f.e.

Any casting or machined metal in the Tesla would have to be flawless internally.

Only rotor. And that is not that hard.

At 18K, ball bearings would need forced lubrication, as the centrifugal force would strip them of lube very soon

Or you should use closed ball bearings. Also, centrifugal force for bearing much less that for the rotor, since diameter of rotating parts is much smaller. Vaccum cleaner turbines runs at 20-30K RPM, and ball bearings perfecly OK without forced lubrication for many years. SKF gives limiting speed for randomly taken W628/8-2Z bearing at 45K RPM f.e.