It is well documentd that win 8 cannot be unstalled. It was covered in some detail in the early days still on this site.
It is presumed that you have already backed up your files and settings to an external drive? ( I found windows easy transfer worked well for me ) The boot manager should be removed after not before. If your win 8 enterprise is working use the disc management consul, from there try removing the partion. You will be able by the same method to reinstate the partion at a later date.
If this fails then you can try installing Win 8 Realease again. then remove it then remove the boot manager.
Another option if you have another working computer is to put the problem drive in it as second drive. this again useing disc management should allow you to format the partion