Sure if they held such a belief. However narrating a quote from someone and holding the same belief are different.
Well there is quote from Mullah Majalisi were he accepted that are 17,000 verses in the Quran and he authenticated it. The 12er Shia scholar are silent on the issue. They do not draw a judgement that the current Quran we have, whoever considers it incomplete is a non-Muslim. Are you really going head and saying such people deserve la'na? Or are you thinking in your mind that the 12th Imam's Quran is real complete version and that's the one which complete?
Well its a start...
Prophet Yusuf's (as) brother were his enemies but there is no proof that he cursed them. So likewise, I will have to accept what is sunnah and not what our conclusions say about who deserves la'na.
What were the actions of Caliph Umar that leads you to believe it was him? Why not Caliphs Abu Bakr or Uthman? Or AR ibn Awf? etc?
The reason is your sect believes Umar killed Fatima (sa) which if it was true makes him even worst than Yazid.
My homework then...
You are aware he lived in the time of Al Qami who worked with Mongals and told them to invade Iraq. In return they Mongals killed over 1.6 millions Iraqis. Then in return Al Qami was promoted as well.