What's your hunch on how solidly acorn Richard Garfield, PhD is? Like on a scale of Uktabi Kong to More or Less, which is he closer to?

Very solidly.

What made Uktabi Kong an un-card at the time? To me it seems completely fine within the rules.

It was more the flavor than the mechanics.

Uktabi Kong is a great example of an Un-card that would be Eternal if it were printed now, but still feels very much like silver border - correct? The name, the art, the flavor text, and one of the abilities are referential jokes. However, nothing is mechanically outside the realm of black-border MTG, not even the "joke" ability.

Here’s a different way to think about this. What if we took every card with a silly card concept/name/art/flavor text in all of Magic and made them acorn. Would that make the game better? Would having cards that you could play in a tournament, but out of bounds simply because it has humorous flavor be upside for Magic? I believe it would not. And yes, Uktabi Kong would be eternal if it was in Unfinity.

Can Un-cards that don't break or even bend the rules of Magic - e.g. Uktabi Kong, The Cheese Stands Alone, etc. - never see print in a black-bordered set? Or has R&D just chosen to avoid it?

We’ve made an equivalent version of The Cheese Stands Alone in black border. Uktabi Kong’s flavor we probably wouldn’t do so blatantly in black border. : )As time goes by, more and more things that were once silver border become black border.

Aside from art/flavor text, what makes Uktabi Kong an Un-card (i.e. something that must be silver border).

We would never pair that flavor with those mechanics on a black border card. If you ignore the flavor tie-in, all the components are doable in black border although don’t work together without a flavor connection.