Is the belief of distortion of quran kufr? How scholars interpret narrations hinting about such? What should a common Muslim do questioning his faith?

This issue got my little attention when I first watched a video sent by my shia friend of a scholar named Hassan Allahyari who holds the view that the quran that we have in our hands is distorted and original quran will be brought by the hidden imam i.e. imam mahdi. I already knew that this subject is used as a weapon against each other by sunnis and shias. Sunnis start first by accusing them of belief of tahreef doing takiyya and shias in reply don't remain silent and bring narrations from the sunni books hinting as though it's a distortion. The web is full of this fighting stuff by both sunni shia sites discrediting each other. My question is now is there is no denying that these kind of narrations do exist, in both sunni and shias books even in bukhari and muslim and other sunan books as well. Here in Pakistan sunni scholars too admit this ,though some reject it completely despite of their authenticity and others make taaweel. Majority shias also disown this belief saying this is a propaganda and false allegation against shia as shias have always been a subject of hatred. Now here in subcontinent a scholar named Hassan Allahyari gained popularity and started disseminating this doctrine of distortion openly and people have started believing in it. My question is what should a common Muslim react towards these and view or doctrine it should hold especially when this era of chaos and secterianism where these kind of heated controversies makes him question his faith? And why are sunni so dishonest by maligning and falsely accusing the opposing party of tahreef when their own books are full of this stuff?