Looks like you have the job almost cracked
A good tip to crack open a stubborn bleed nipple is to use a ring spanner and hold some force on the nipple as if to loosen it, with the force applied repeatedly tap the end of the bleed nipple with a hammer, this normally does the trick.
99% chance it's the sliding mechanism on the caliper that is seized not the piston, this can normally be freed off by a bit of cleaning up and a bit of grease. Sometimes the pads become that tight in their location that they don't want to move, once again a good clean up normally cures any problems.
It's good practice when bleeding only push the pedal half way down...it never travels to the floor under normal use so that part of the master cylinder could have corrosion on the walls which can damage the seals and cause fluid let by which would be evident by a sinking pedal after the bleeding.
(The above mainly applies to older cars with cast iron master cylinders, modern master cylinders have alloy internals to prevent corrosion, but none the less a good practice)