Upon further investigation the brake disc is not frozen. One of the broken bolts is jamming the rotation.
With this relegation I can only inquire as to the probable cause of the bolts shearing.
Should I replace the ones on the right side also?
I worked for BMC and BLM in dealerships through almost the whole life of the MGB and I never seen the bolts shear off on the disk to hub. My guess is that the disks were replaced by a mechanic who used a air wrench to tighten the bolts and thus he over stretched the bolts. Too many mechanics use air wrenches to tighten bolts that should be torqued.
I taught engine theory and engine rebuild in two major commercial mechanics schools and we stressed to "Take things apart with an air wrench and build with a torque wrench". But I see many mechanics today build with air wrenches.
I take my car today to get tires put on and I make sure I am standing there when they put the wheels back on so I can tell the mechanic to just snug the wheel lug nuts as I will torque them at home.
I have seen mechanics set their air wrench on a low torque and then use a torque wrench and that is ok or use a 3/8 drive air wrench and then use a torque wrench. No, I don't torque every bolt on my car. But I do torque all the bolts that have my life in their hands.