In discussing the inter-relation of axes with @Laurence Penney
it struck me that there's a conundrum here that might need to be resolved sooner rather than later (although it's possible others have actually already worked through this).
I posit that in this new continuous-axis world there's another approach that might be more intuitive for users: the weight axis maintains set width; this would cause a shift in apparent width, but the width axis would become literal (as the weight axis is); a uniwidth toggle would become redundant; and any apparent-width compensation would be done manually via the width axis. Of course this gives the user more power, but also arguably a responsibility that might backfire; the type designer's expert judgement of the relation of weight and width becomes lost, and is difficult to replicate.
So maybe an interface could provide a choice between Set Width and Apparent Width...