What is the religious ruling on diminutizing names for endearment, such as diminutizing "Nahla" to "Nuhayla," and "Khadijah" to "Dujay," and "Muhammad" to "Hammudah"?

It is permissible to shorten a name or reduce its letters, as long as there is no mockery or disparagement in doing so. This has been mentioned in the Prophetic Sunnah (definition), such as the Prophet (peace be upon him) shortening Aisha's name to "Ya A'ish" and Anjasha's name to "Ya Anjash."

However, it is prohibited if it becomes a form of calling by offensive nicknames or disparagement, which is forbidden in the Almighty's saying: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers."

Therefore, there is no harm in calling "Nahla" as "Nuhaileh," and "Khadijah" as "Dujayy," and "Muhammad" as "Hamouda," if it is done in a playful and affectionate manner.