i dont feel like reading through a bunch of blabber so take this for what its worth.
1. indicate the tooth runout of the EM, on a 1/8 cutter you really need .0005 or less.
2. forget about all the HSM BS for for now, its all great in theory. But theory and the shop for need to meet somewhere in the middle.
Try 7000 rpm at 25ipm .06 Z depth of cut per pass .01 radial, id run coolant. put a .003 corner rad on each tip. More or less take a light lapping stone and in your hand go over the each corner 2 times. If this eliminates your chatter on roughing the corner out but it still chatters in the the last pass. You might want to try a smaller EM to finish the corner. Or if your tolerance allows it, try programming an additional .005-.01 rad in the corner so the EM doesnt Gouge into the corner as much
Van Fleet Precision Machining
http://www.vanfleetprecision.com/