I dont see that the fact that seh might have pokcets in her day dresses (for convenienece) means that she's behind the times. (when did pockets "go out of date please")

It seems very sensible to me not to give up something which is useful just becuase others had done so.

If she was still wearing panniers and heavy silks or satins etc when everyone else had changed to the light muslin empire line gowns that would be behind the times... but I dont see any indication that she's doing anything liek that. I woudl say that her evening gowns are fashiable but not "outre" and that they are elegant and well made not vulgarly over trimmed like Mrs E's.
and I dont see why she could not buy dress lengths in London where there was a good choice in them (better than Fords) and have them made up in Highbury. Otherwise she woudl have to go to London ot be fitted, and that WOULD be extravagant...