What happens if the XBMC does not start and the default.XBE is overwritten with the XBMC one (I plan to use the XBMC Shortcut utility)?
If the default.xbe started by the softmod is not a dashboard, it is possible that you need to fix it by connecting the xbox harddisk to a pc again. That's why it is important that you have a backup of the xbox eeprom.bin file. If the softmod can't locate a default.xbe, it will search alternative locations like the dvd drive for a valid file.
The file called shadowc_off.bin can be any file with that name or there is a dedicated one ?
The softmod only checks the existance of that filename. The contents of the file don't matter. The location is different between nkpatcher10 and nkpatcher11. If you have a NKP11 folder on your E partition, you will need to place the file there.
I've read that for having a shadow C write enble, I have to replace the shadowC.IMG with the one on the computer crated under NDURE\C\SHADOWC. Is this because the SHADOWC.IMG on the E folder is read only ?
What happens if I write a wrong file ?
With a wrong file, you will end up again with an unwritable shadowc partition. It is not the shadowc.img file on the E that is "read only", it is the one on the C in the "shadowc" folder. Even if that file contains nonsens, a format of the shadowc partition should solve the issue. If the file is only 5MB long, the shadowc partition will only have that size as well. (and the M$dash files won't fit on it.)
In a perfect world, you indeed don't need to touch the shadowc.img file. If you can't read and format your shadowc partition, you will need to fix it.