Isn't believing in the "fullness" of the Quran a aqeedah of the Sunni faith?
do your research and you will find out what Shia muslims believe about the Quran, amongst other central Sunni beliefs.
Yes, this is the grave sin, and some also believe in the "createdness" of the Qur'an.
Shias believe that revelation came to Muhammad by mistake, and that Ali ibn Abu Talib was supposed to receive the revelation.
Actually not all of them do.
Shi'as believe that the standard of Immamat is higher than the Prophethood.( Hayatul Qaloob Vol. 3 pg. 10)
Shia's believe that Angels bring WEHI to the Immamas:every Friday, they go to MIRAJ.( Asool-e-Kaafi pg. 135, 155)
Shia's believe that there is an Alteration in the Qur'an. A big part of the Qur'an is missing, they claim that the Qur'an is also corrupted and there were changes made in it. The Shi'a Quran is different from the actual Quran. (Sani Comt. Asool-e-Kaafi Vol. 6 pg. 75)
Shia Immams got higher status, that can't be achieved by the Angels nor the Prophet. ( Khomeini . Hakoomatil Islamia pg.52)
Shi'a beliefs of 17,000 ayats (verses) in Qur'an ( Asool-e-Kaafi pg 671)
Hazrat Abu Bakr and Hazrat Umer were never became true Muslims according to the Shi'a. ( Jal'l Ayuoon pg. 45 )
After Muhammed passed away, all the Sahabas left Islam except for three. ( Qur'an Kaafi Vol. 3 pg. 115 )
And I'd disagree with that, but the question is what in Orthodox Islam do they disbelieve? They believe a lot of additional things, which are serious infractions, but in what do they disbelieve? You have to start listing things that remove them from Islam.
Peace