i dont know what the 1/2 inch plug is???? The radiators are about 20yrs old fairly thick metal. Can a new vent be fitted as they stand. Present vent is on the inside close to wall.
have you sorted this yet?
Before I can make some suggestions is there any way that you can take a photo and post it of the bleed nipple.
The most common reason that the bleed nipple key does not properly locate and slips is that it has a "shoulder" that needs to be ground down first. Worth a try rather than drain down and remove/change the rad