what you need to think of, is what causes brakes to squeal...one reason is movement within the caliper set up...a bit like when rubbing your finger over the lip of the wine glass...
bearing in mind i have no idea why your brakes were imbalanced or what was done to fix it but as an example..
if your pads had a build up of corrosion where the metal edges of the pad snugly fits into the caliper body..that corrosion causes the pad to stick and prevent the essential movement for the pad to be pressed evenly and hard to the disc..to fix it, the pads were removed and corrosion build up cleaned off to allow even pressure both sides of the disc on each caliper.
end result could now be the opposite of what you had..now when you brake there is a very small movement where the pad fits snugly to caliper body...causing a high pitch squeal...
with that in mind...
that movement could be the pad into caliper at any point there is metal to metal contact, or any retaining pins where they contact any metal, or spring clips where they contact any metal...be it spring clip to pad or spring clip to caliper body...hopefully you see what im trying to say.
so copper grease should be used not only on rear of the pads, but anywhere there is metal to metal contact.
if this doesnt cure it then sometimes the only answer is to replace the pads and/or the fitting kit.
if brakes were uneven due to a problem with the sliding mechanism of caliper, then that could also be where excess movement is, now they've been cleaned up,..in which case better off using type of grease you find in cv joints to lubricate the sliding pins rather than copper grease,