I use (x64) GimageX v2 on my WinPE_x64. New version has corrected old major bugs.
Start gimagex.exe anywhere as Administrator. (Not on drive X:)
Tab Mount: I select source first, then mountpoint. Check on 'Read and Write'.
Mounted a boot.wim successfully.
Manipulated for testing name of file bootmgr in Mountpoint to bootmg_____r.
Now close Explorer for safety.
Check on 'Commit Changes'; Unmount.
My E:\WIMMOUNTPOINT is after this procedure emty.