Those two at the bottom of the bore would actually be the easiest to get moving, with the geometry of the crank position. Turning force on the crank would be multiplied many times with pistons at BDC or TDC. But the others at mid-stroke would have the least mechanical advantage and would keep the crank from turning. Once disconnected from the crankshaft, each piston is an independent problem and usually will move without too much problem. On mine, even that one that was really stuck moved until it got to the top of the bore where there was too much rust and crud. I suppose I could have wire brushed the bore like you did and had better success, but I got it done with the brute force method.
