The kind of "repair" Testdisk can do on a MBR is limited, it will only "fix" the contents of the partition table (and it has anyway to be seen specifically if the small partition that RMPRPUSB/Easy2boot usually makes with ID 0x21 has been fixed and/or is actually needed in your case).
If there was - for *whatever* reason - a corruption of the Code of the MBR (which BTW in the case of grub4dos extrends for several sectors after the MBR) Testdisk won't be able to do anything about it.
It is strange that "synchronizing" the NTFS bootsectors results in a "cannot access" kind of error, I mean it could be the symptom of a more serious problem.
If you have saved your payloads and whatever customizations you had before, yes, I would rather re-run E2B and "start from new".
Once you have it working, it is a good idea for USB sticks (even if yours is a tad bit on the "large" size) to make a dd-like or "forensic sound" image of the whole device, this way you have a "way back" in case something "goes astray", you can usually comprss it in a .7z or .zip and won't take that much of space on a backup device.