Here is another test with a different powered-catalyst pellet having roughly half the resistance.
Similar overall results were observed after adding acetone (which caused a rapid decrease in resistance possibly due to hydrogen—although I cannot rule out other causes), but this time the
jar vessel, which I had to replace due to breakage, was apparently better sealed and eventually I had to open the lid to let air in. This caused resistance to increase much faster. The final resistance matched the starting value of 2.85 Ohm. The minimum value at 13V was 1.97 Ohm.