Thank you very much for the feedback about LTSB ... this is what you could try :
- mount your ltsb (the offical one without modification)
- integrate only your updates
- save this iso (without modifying anything else)
- test your iso, in a virtual machine, to check if all your updates have been correctly integrated
==> Keep this iso for future use ... I'm doing like this ...
- Then in a second time, mount this new iso (the one with all updates integrated) ... and apply your wccf.