Doubting the success rate of an ‘agreed upon’ approach would be doubting the Word of guidance of Allah. You are implying ‘agreed upon’ approach would yield only in fairy tales, such as moon sighting, which supposedly may or may not work.
‘Agreed upon’ approach is the guidance of Quran. Quran tells us whoever follows Allah’s guidance is set up for guaranteed success!
39:23 God has now revealed the best Hadith, a Book fully consistent within itself. IT MARKS OUT BOTH WAYS (TO SUCCESS AND FAILURE) repeating its Messages in diverse forms…
42:46 They will have no patrons to help them besides God. One who goes astray by violating God's Laws of guidance, For Him There Is No Way (To Success). [42:44]
‘Moon sighting’ – the conflict issue - is not the result of an ‘agreed upon’ approach guidance, but a clear consequence of mutual rivalries among the groups. Take for example, Deobandi vs Barelwi, 24 vs 26, Shia vs Sunni, Hanafi followers vs Shafi followers, and such. The problem actually runs much deeper, they are predisposed to not agree to pretty much anything the other proposes. More often than not, their issues are personal disputes and conflicts. They will never be guided until they come back to the Qur’an and genuinely look for answers ONLY in the hadith of Quran.
2:213 Mankind were one single community (but selfishness divided them). So, God sent Prophets as bearers of good news and warnings. He sent down with them the Scripture in absolute Truth that it might judge between people wherein they differed. BUT AGAIN, OUT OF MUTUAL RIVALRY, THEY DIVIDED THEMSELVES AFTER ALL EVIDENCE OF THE RIGHT HAD COME TO THEM! So, God shows the lighted road to those who would acknowledge the truth and thus resolve their disputes. And God guides to the straight road him who wills to be guided.
[Satan, the selfish desires, had rent asunder the unity of mankind. 2:36, 10:19. Using the perceptual and conceptual faculties in all humility help people achieve guidance, while selfishness, arrogance and blind following lead them astray. 2:170, 4:88, 6:56, 7:173, 7:179, 17:36, 27:80-81, 30:22, 37:113, 40:34-35, 41:17, 56:79]