Man, I miss a couple days in this thread and so much happens I'm totally lost. ;-)

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So I'd like to ask for your opinions again: should I do more work on perfecting her body rigging, or should I put that aside for now and start on an expression system (meaning a collection of morphs and a set of dials to drive them in a user-friendly way) for Antonia?

Personally I'd try to get whatever I was unhappy with, rigging-wise, taken care of and out of the way first.
However, it can also be good to move on to other things and revisit older problems later with a fresh set of eyes and ideas.