Punishment for Zina Between the Qur’an and Sunnah

Assalam-o-Alaikum I am a born Muslim fortunately but my problem is that I see so many sectc in Muslims and I wish to find the right one. I try to understand Quran but I am amazaed when I find some laws in Sharia which are contradictory to Quran's orders. I will put one example hereWhen Quran tells us the punishment for "Zina", it does not diffferentiate among sinners. No distinction is made among married or unmarried "zaani/zaaniah" and the punishment is stated as lashes.In our present sharia however it is believed that this punishment is only for unmarried sinners and for married sinners it is stone to death. The basis of this discremination is from the book of Hadith.My problem is that I believe that Quran is the only 100% authentic set of rules from Allah. Whtever else is, it is not sure that it has reached us as it was original. Even contents of books of Hadiths are not 100% sure as no one including Allah, Rasoolullah or the compilers of books claim those to be 100% sure.Definitely whatever is from Rasool, it is to be followed but after it is pro ved to be 100% sure from Rasool. For me it is difficult to believe that Rasoolullah made this ammendment in the punishment for Zina. If he would have done, he must have made arrangement for the documentation of any such ammendment. But as I know, this was not recorded till the compilation of Hadith books. Such an important matter, an extension or ammendment in the order of Allah's words, if from Allah or Rasool (of coure Rasool did not do anythinng himself rather he obeyed Allah), why it has not been recorded by Rasool, Khulafa-e-Rashideen or any responsible person but only by a scholar after more than two centuries.Can any scholar/alim please answer my confusion? May Allah lead me to His true way and the real Islam.Regards