the "flat Seals" are actually square seals, and are designed that way.
the inner face of the seal kinda sticks to the piston.. when you squeeze the lever, the seal BENDS... when you release the lever, the seal makes the piston move back to it's stating point. This is how all disc brake calipers work, including Car and Motorcycle types.
the seal is the "return spring".
any other springs found on the caliper are anti-rattle, anti-squeal. Not all calipers have those rather weak springs.

i rather doubt the caliper leaked oil onto the disc... it was likely from some external source.Chain OVER-lubing or messy lubing of components usually cause the oil contamination of brake discs and pads... cleaning the pads will not remove the oil that has leached into the porous pad material... trust me.. many have tried, and it fails to work.
Just replace the Pads.
the discs can be cleaned.. it involves brake cleaner as a first step, then Isoprpyl Alcohol, and lots of good paper towels... some even use flame heat, but that can warp the calipers..
simply wiping the discs will not be good enough... all those holes in the discs hide oil well.
many just buy new discs when they buy pads.

Semi-metallic pads are the most popular, with decent wear rates, and fairly quiet.... "organic" pads stop the bike quicker, but wear out quicker too... full metallic pads last the longest, but have a nasty habit of squealing like a banshee at times, and are just generally noisy all the time... they also wear out discs faster than the other two varieties do.... .

magical "Brake disc and pad coatings" just make another mess, from what i've seen and heard.

brake quiet is a back-of pad coating that is for car brakes... i have it in my garage, but have never applied it to a bicycle pad, ever.

if you get new discs, CLEAN THEM Before installing/using, ok? they are sometimes hit with a non-rust covering before shipping... Most Disc Makers refrain from the coating now.