After all this, is there any consensus on the actual hardware?

Is it possible that the two integers are just

- a clock prescaler

- a conventional UART divider

Almost, you need to get the fractional decision in there somehow

The CD4089 is just a sync counter with a selector on the outputs. It doesn't "multiply" the clock in any way.

Yes, but it can give a varying density spread swallow pulse enable. Thus it gives a means to enable a /N or /(N+1) counter

By varying when that decision is applied, you can on average, get some average adder between +0 and +1 - ie a fraction

" as any edge has to snap to a 42Mhz slot."

That's the clever bit I don't get.

The edges have to be aligned to the clock, so the fractional part is an

**average over many bits**.

None of the bits are actually of exactly the desired baud time, some are slower and some are faster, but any edge should (ideally) always be at the nearest (1/42M) time slot.