Some words in the Arabic language have many diacritical positions whereby it’s possible to put fatha, kasra or dhamma like the word “siwa” and “ghayr.” How then do we read them if we come across them in the Quran? Do we read them with fatha, kasra or dhamma or we are required to read them with a specific vowel? Thank you. الأحوال الإعرابية في القرآن

Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
As to what follows:
With regards to answering your question we say, success is by Allah Almighty.
Words are read as they appear in the Mu’haf because any specific vowel that appears on a word depends with the context of the speech.

Your brother,
Dr. Khalid al-Muslih