Yes, Hydrogen Peroxide the same stuff you can buy at Costco, Sams or any drug store. You probably already have some in your medical supplies as it is a mild antiseptic. It is usually 3%, but strong enough to make the vinegar a super oxidizer that will dissolve lead or break it's bond to the underlying metal.
850 does seem a little too hot, but whatever works. Have you checked your pot with a thermometer to see if it is really at that temp? 700 doesn't work for me either, since I run two molds simultaneously. I typically need it in the 725-750 range to keep the alloy flowing. I also don't worry about the alloy mix as I shoot range scrap, typically is in the 10-12 BHN range, but since it is powder coated I could shoot pure lead at handgun velocities without worry. Hope some of the suggestions work for you.
BTW: I have used electrolysis for removing rust from steel/cast iron and that works quite well, but I am not aware of a process for aluminum. I wouldn't recommend ultrasonic cleaning on aluminum either or at least not for any duration.