how to allow span inside p (ckeditor)
Did these links not help:

https://stackoverflow.com/questions/18349048/ckeditor-strips-span-and-style-attributes

"If you are using the CKeditor module then there is an option in Advanced Options that is also mentioned in the module homepage where you should set"

or:
Prevent WYSIWYG+CKEditor from stripping html classes
"There is an issue with CKEditor 4.1+, which has a feature called Automatic Content Filter (ACF) that will automatically strip attributes not defined for the editor"