〈 Berger | Dillon 〉

why would the electron generate radiation while orbiting? also, if electrons don’t orbit the nucleus, but they also don’t ever collide with the nucleus, then what are they doing all the time?

Because accelerating charged particles radiate. What they're actually doing is existing in a superposition of states that can be approximated to some extent (which breaks down fairly quickly) with a classical conception of orbits.

if they’re not accelerating, does that mean they are staying still? if they were moving with a constant nonzero velocity, then they would eventually separate from the nucleus, right? So each electron’s velocity must be zero. Is that right?

Classical conceptions of motion break down at the quantum level and it is not precisely useful to think of bound electrons as particles with classical position and momentum.

Have we observed radiation generated as a result of acceleration due to gravity? Just wondering if following a spacetime geodesic is really acceleration relative to the underlying field.

indeed that's what gravitational waves are!