>one metal always sets the value of the other making the bimetal system redundant.
I don't think you are correct here. I can't see how gold would "set the value" of silver. Furthermore you're assertion is totally out of step with Marx who asserts that the value that both gold and silver bear as commodities is ultimately measured in socially necessary labor time. This is what, according to Marx, sets the value of the two commodities, silver and gold.
>The general trend (which marx adds in a footnote isnt historically universal) is towards the higher value because (i assume) ease of transportation and storage.
This is a reasonable explanation for why Marx says in footnote 4 that the metal with the over estimated value would alone serves in reality as the standard of value. The problem with your idea here is that Marx gives historical examples of bimetal states withdrawing currency in the overestimated metal and exporting it. This act of exporting reverses the state of the ubiquity of the commodity that would provide easy transportation and storage.
>the footnote you are citing is in reference to when one of the metals gets out of swing of the other, not that gold in a bizarro country is somehow less valuable than silver.
I understand the matter is when metal A gets out of sync with metal B. My confusion is that Marx makes a normative claim in footnote 4, ("the over-estimated metal which alone serves in reality as the standard of value.") which is contrary to his statement later on, (“with the development of material wealth , the more precious metal extrudes the less precious from its function as measure of value. silver drives out copper, gold drives out silver.”). Of course gold remains more valuable than silver but in some cases (as in the case Engels refers to in the letter I cited) states set the exchange ratio between silver and gold in such a way that overestimates the value of silver. I will give a concrete example. According to Engels the ratio of silver to gold in Germany in 1882 was set by the state at 15½ pfennig silver = 1 pfennig gold when in reality Engels pegs the actual ratio at 17½ pfennig silver = 1 pfennig gold. Then Engels claims that "Silver loses more and more each day the capacity of serving as a measure of value, gold retains it." Gold here is clearly the metal with the under-estimated value but Engels asserts, seemingly contrary to what Marx has said in footnote 4, that this under-estimated metal is the only one of the two which can still really be used to measure value. I don't doubt that there is an explanation for all of this that is perfectly consistent, Capital is after all a book which defenders of the status quo are constantly looking to poke holes in and I've never heard a criticism of Marx's understanding of the bimetal currency system. Having said that I still don't know what the explanation is.