you are slightly mistaken about the way sunnis view the quran and abrogation. there are 3 types of abrogations: 1. verse and ruling both abrogated. 2. verse abrogated but ruling stays. 3. verse stays but ruling abrogated. the third kind remain in the quran today as well ofc, for example the verses of khumr being permissible under certain conditions. the first and second types are the ones that are mentioned in the ahadith. but this conversation has slightly derailed. i hope you dont mind my going back to the original point.
question was, how can we trust the shia narrators/versions of ahadith? i will do you one good. i will stop presenting the sanad argument. and even though you said i believe in the quran because of sunni hadith, i dont. ofc not. anyway, i will ask the same question about the sunni narrators. how can we trust them? so that we have no reason to trust either of the narrators. fair enough? all we have and believe in is the quran.
so the next questions that comes up are two: 1) which version should we trust and accept, the sunni or the shia? and 2) without relying on either shia or sunni hadith, how can we justify different variants of the quran available today or how do we know which is the actual quran?
i know you might think i am repeating myself. but i am not. also, i want to outline one possible response from you before hand. you might say that the quran itself says that Allah wants to rid the ahlul bayt of rijs. to purify them. and the ahlul bayt tell us that the quran which is prevalent among the masses is the actual quran. but the problem with that answer is that the verse of purification is for the 5, or the ahlul kisa, not for the remaining imams. and the only way you know that the remaining imams are included in this verse is because of ahadith narrated by shia narrators. which we decided, along with sunni narrators, cant be trusted. and hence this response to the second question doesnt hold.
P.S. i dont believe uthman arranged the order of the surahs. i believe the order of the surahs was decided by the prophet pbuh, not by the companions. as for the hadith in abu dawud and tirmidhi which basically says uthman placed surah taubah and anfal together of his own, it is a weak hadith.