1. How do you know what you have mentioned about the tombs of Abraham, Sara and Isaac in 'Tombs of the Patriarchs' being in Israel? What is the authority of your conclusion ?
Tombs of the Patriarch https://www.youtube.com/watch?v=-c_PkRMqPH4

The Tombs of the Patriarchs: Abraham, Sarah, Isaac and Rebecca. (Hebron)

2. Are people worshipping these graves?
(No, I have not heard from anyone who have visited the holy land, worshipping these graves)

3. If grave worship in your view, is the reason, then why is our Messenger's tomb intact?
(I do not have sufficient knowledge to answer this question.)