It seems that there's an 'unexpected error' at the importation of the modified style even after changing the characters between the id and title fields...
Like this for example :

Cellmodified (title field)
modified (id field)

Here is my modiefied version of the style 'Cell.csl'. I just add the line that encodes the 'et al.' in italic and renamed the title and ID fields.
https://hastebin.com/lovabobixe.xml
Thank you again !

should work, but it might not like the id. Instead try
https://www.zotero.org/styles/cellmodified