The brass dust is most likely coming from the left flipper plunger stop. There is a brass part in it that eventually wears completely out, and then the flipper starts getting stuck in the upright position. No need to worry, pretty standard wear item for newer sterns. Maybe just order a spare and have it on hand for when the flipper starts sticking.

As far as the squeaking goes, good luck. Sometimes you can stop it, sometimes not.

Mine squeaks intermittently. I tried to oil it, but that did no good. Luckily, it does not squeak more than it does. Turn the sound up on the machine!

What are the brushes, and are they in a different area than where you put the oil? What kind of cleaner did you use? I dropped some oil on the rod that holds the weights right between the weights and the housing. Did you do something different?

Just for kicks, check your left flipper coil stop. My brand new ST Pro had brass dust on top and near the shaker motor. Turned out to be the coil stop was loose and spinning on the bracket.

looks like a pretty standard mabuchi 540 can type motor (or johnson, same-same) easily replaced. hard to tell its exact size-scale from the photo, 280's 380's and 400's are built the same but smaller.

Thanks for the explanation. I'll take a look and give it a try the next time the squeaking is bothering me.