It doesn't matter what ever excuse you come up with or which ever way you turn it, but you can't turn back the clock. What Umar said, did and how he responded is what he is and will be judged by. The matter is clear and you can carry on looking for excuses. I completely understand that Umar desperately needs to be protected here.
Allow me to focus on that portion of your comment, "which ever way you turn it", and see whose ideology fails from all sides.
1. Umar (ra), according to you, not only disobeyed the Prophet (saw) but also stopped others from bringing a pen and paper. For not taking a different course of action, Imam Ali (ra) is just as guilty. And if Imam Ali (ra) was subdued, your problem becomes manifold.
2. The Prophet (saw) was alive three days thereafter. Was Umar (ra) standing next to him the whole time to ensure nothing is written?
3. We all know what Shias hope the Prophet (saw) would have dictated, namely the successorship of Imam Ali (ra). Why make all this noise when you claim that the announcement was already made at Ghadeer?
4. In Kitab ul-Irshad, Shaykh Mufid makes a huge blunder in a haste (much like yourself). He narrates what happened after the Prophet (saw) asked the Companions (ra) to leave him alone.
"When they (the people) had left (the room), he (the Prophet) said: 'Send back to me my brother (Ali) and my uncle (Abbas).'
They sent for someone to call them and he brought them. When he had them sitting close, he (the Prophet) said: '
Uncle of the Apostle of Allah, will you accept my testamentary bequest (wasi), fulfill my promise, and carry out my religion?'
'Apostle of Allah, your uncle is an old man with the responsibilities of a large family,' answered Al-Abbas. 'You vie with the wind in liberality and generosity. You have made promises which your uncle could never fulfill.'
Then he (the Prophet) turned to Ali ibn Abi Talib, and said: 'Brother, will you accept my testamentary bequest (wasi), fulfill my promises, carry out my religion on my behalf and look after the affairs of my family after me?'
'Yes, Apostle of Allah,' he (Ali) replied." (Kitab Al-Irshad, by Shaykh Mufid, p.131)
After the Prophet (saw) had allegedly declared Imam Ali's (ra) successorship at Ghadeer, why would he (saw) then offer it to Abbas (ra)?
This is the level of Shia blunder, thanks to extreme misguidance as a result of blind hatred. They claim that Imam Ali (ra) was already appointed the successor at Ghadeer. Then, they further argue that his successorship would have been written down as a will. However, when no such thing was done in the days that the Prophet (saw) was alive, they further fabricated a follow-up situation in which they allege that the Prophet (saw) offered the leadership to Abbas (ra) before Imam Ali (ra), totally ignorant of the fact that this follow-up scenario totally discredits their first alleged claim which is that the fate of leadership was already sealed at Ghadeer.