Is there any easy way to see what inputs are assigned to a bus above B?

With a lot of inputs and variable assignments depending upon the production, it's kind of a pain to right click on every input to see if busses C-H are checked or not. Doubly so when youre live and some unknown source is heard on an invisible bus and must be located.

I'll make a feature request about the audio section GUI as there are a lot of things that might improve usability and intuitiveness.

You have to keep in mind that vMix didn't really used to deal in audio at all beyond the single Master output.

Buses A & B were added with the toggleable Master in v18, November 2016.

The others were added in v23, December 2019.

When vMix implements changes like that, they generally do so very slowly and incrementally. That, of course, makes a lot of sense, as it's better to start small & make sure things work so you can narrow down what's broken if it doesn't.

From where they started, they've come a really long way in a really short time.

There's room to improve it quite a bit, as you've noted, but keep in mind that it's still in its infancy compared to the other aspects of the program. And change is generally slow and incremental.