I think with more time I could eventually build this into something that works at R136/15, but I don't really have the energy.
The idea is to backfeed the extra mercury from the reaction 3AuHgAg -> critellium + Hg into the upper copper production line and turn that copper into gold, combining it with the silver made from tin.

This would be sufficient to turn 15/17 coppers into silver - the remaining 19 Hg would come directly from the 6/17 CuHg₂ that get purified.

There would of course be some ruined critellia that wound up with one gold or another replaced with silver, but they would get pulled off the output and merged into a chain without causing issues - the spare mercury would still be backfed, and the rate at which real+pseudo critellium is made would not change (ideally), so once the backfeeding got done there would be no more ruined outputs.