al-Salamu `Aleykum,
This is an old topic I translated, I will add additional paragraphs which I missed:
al-Masa'il al-Sarawiyah by al-Mufid (d.413 hijri) page 78 - the ninth issue:
[There is no doubt, that what is between the two covers of the Qur'an, its whole content is Allah’s words and revelation (1), there is nothing from the words of mankind in it and it is the majority of the revelation (2).And the rest of what was revealed by Allah most high is with the guardian of Shari`ah, the preserver of Ahkaam, none of it was lost (3). Although the one who has gathered what is found between the two covers today (4), has not included it along with what he collected (5) for some reasons that caused him to do so, such as:His inability to know parts of it.And: His doubt and uncertainty concerning it.And: What he purposely kept out of it. (6)Ameer al-Mu'mineen (as) had gathered the revealed Qur'an from beginning to end, and he compiled it the way it should have been, placing the Makki before the Madani, and the abrogated before the abrogation, and he placed everything in its correct location. (7)This is why Ja`far bin Muhammad al-Sadiq (as) said: "By Allah if the Qur'an was recited as it was revealed you would have found our names in it just as those before us were named." (8.)And he (as) said: "The Qur'an was revealed four parts, a quarter about us, a quarter about our enemies, a quarter are Sunan and examples, and a quarter is obligatory duties and rulings, and for us Ahlul-Bayt are the best parts." (9)However, it was authentically narrated from our Imams (as) that they were ordered to recite what is in between the two covers, and to not exceed it by addition or subtraction, until al-Qa'im rises, then he will recite for the people the Qur'an as Allah revealed it and as was gathered by Ameer al-Mu'mineen (10).]Footnotes by me:
(1) He means all that is found in our present Qur'an are true words of Allah, obviously he doesn't mean this is ALL the Qur'an. It means he doesn't believe in additional Tahreef.
(2) Notice he says "Jumhour" meaning MOST of the revelation, not all of it.
(3) Meaning the rest of what was revealed is with the guardian, he most probably means the Mahdi.
(4) Meaning the first three Khulafa' that collected the Qur'an we have today.
(5) He claims these Caliphs never included the "rest of what was revealed" for their own personal/political reasons.
(6) As you can see, he claims the Caliphs didn't know parts of the Qur'an, so they missed them, they had doubts about other parts, so they skipped them, and finally they intentionally removed some parts.
(7) He's telling us that the correct Qur'an that contains everything was only gathered by `Ali, and obviously we all know the story they narrate about the Caliphs rejecting his Qur'an.
(8.) al-Mufid believes the names of the Imams are mentioned in the correct Qur'an.
(9) al-Mufid believes that half of the Qur'an talked about the Imams and their enemies, this part was omitted by the Caliphs.
(10) He believes the Mahdi will reveal the true Qur'an later, after this nation goes to hell for being misguided by Allah (Astaghfirullah).
This is the additional paragraph, on pg.82:
وإنما نهونا عليهم السلام عن قراءة ما وردت به الأخبار من أحرف تزيد على الثابت في المصحف لأنها لم تأت على التواتر، وإنما جاء بها الآحاد، وقد يغلط الواحد فيما ينقله.
ولأنه متى قرأ الإنسان بما خالف ما بين الدفتين غرر بنفسه وعرض نفسه للهلاك.
فنهونا عليهم السلام عن قراءة القرآن بخلاف ما ثبت بين الدفتين لما ذكرناه.
[They (as) only prohibited us (1) from reciting what is found in the narrations from additional letters -in addition to what is already established in the Mushaf- because they have not reached us through Tawatur, rather it is narrations of Ahaad, and one might make a mistake in transmitting (2).Also, when a man recites differently from what is between the two covers, he risks perishing. So they (as) prohibited us from reciting in opposition to what is between the two covers for this reason we listed (3).]Footnotes by me:(1) He uses the word "Only" here as there is only one reason as to why Shia shouldn't recite based on the narrations of Tahreef in their books.(2) Meaning, the only reason isn't because this Qur'an is complete and free from corruption, rather the only reason is because not all the narrations of Tahreef are correctly attributed to the Imams, so one can mistakenly recite based on a fabricated narration.(3) Notice his consistency, he never says the Qur'an is free from errors, he just warns us from reciting with the additional words found in Shia narrations due to the risk of stumbling upon weak ones.