Spray with LPS type oil lube....maybe WD40...... Use air pressure, don't pry. Take the bleeder valve out and check for obstructions. (That would never happen!) Look down in the hole to see if there is anything in the caliper hole....
That is odd that it pushed in easy and is now stuck.....something simple going on.
Use more air pressure, BUTT BE SURE to put a piece of wood in the caliper to cushion the piston should it suddenly decide to eject itself! NO FINGERS!!!!....or you'll have less fingers when done......
Use brake lube when reassembling.... DOT 4 for fluid.
I have never heard of anyone getting the giant side screw/cap loose. You'll bugger it up most likely.... No need to remove it. Pistons "float" in the housing. Bore can be hand sanded to remove garp but don't need to worry about resleeving them unless REALLY corroded away....(from use of DOT 3 fluid!)
If you need that sort of work.....there is a guy in LA that specializes in Girling stuff.
I believe that the four inner bolts removed will drop the rotor. BUTT you need to remove the entire assy from the spindle anyway to get the rotor off, which will reveal the truth!
Bearing cap has an o-ring in it, so go easy on getting it out....silicon grease when reassembling!
Go baby go!!!! Wait! STOP!!! 
Steve