Did Abdullah ibn Abi Sarh apostatize for a valid reason?

My question is straightforward, If Abdullah ibn Abi Sarh "Genuinly" had a serious doubt that caused him leaving Islam (i.e he wasn't a hypocrite but a true believer) Then how can we justify that his generous and unbiased conclusion which made him leave Islam, still to be called apostasy and treason. We know that he was a scriber of Quran then the incident occured that he was allowed to add some words into quranic ayats by the permission of prophet Muhammad (saw) himself, isn't it a real and thought provoking on itself to doubt Islam and the prophet, I would be realistic and honest if were in place of abdullah ibn sar I would have done the same thing. If you come with the answer that because the allowance of wording in Quran didn't really had that much siginficance in the meaning because at the end of the day it should reflect according to whole notion of Quran and doesn't contridict the quran, then I would ask isn't the Quran already the part of ( Al- kitaab) and its written and recorded along with other ( suhoof and previous scriptures) in "Lawhe Mahfooz", so hence, it must not have any other human's aditional wordings and additions to begin with.