There is one more thing that should be added here – the second part of Sunnah transmitted through akhbar-i-ahad has never been given the same sanctity like the first part of Sunnah transmitted through ijma and tawatur, even as per our classic traditional jurists.
Ibn-e-Abdul Bar has written in his book, that religious status of person negating the first part of Sunnah will be judged, but the second part of Sunnah (akhbar-ahad) is only proof to act on certain matters, it is not a matter of belief in itself. Something similar is said by Imam Shafi regarding akhbar-i-ahad:
وعلم الخاصة سنة من خبر الخاصة يعرفها العلماء ولم يكلفها غيرهم وهي موجودة فيهم أو في بعضهم بصدق الخاص المخبر عن رَسُوْلَ اللّٰہِ بها وهذا اللازم لأهل العلم أن يصيروا إليه
“And the knowledge of the select is the sunnah which is acquired through their reports, which the scholars know and which is not essential for the common man to know. This sunnah is present with all the scholars or with some of them from God’s Messenger (sws) through the information provided by a reliable informant and this is the knowledge which scholars must necessarily turn to“.
(Abu Abdullah Muhammad ibn Idris al-Shafi’i, Al-Risalah, 1st ed. (Beirut: Dar al-fikr, n.d.), 478.)
Secondly, Ijma and Tawatur are modes of transmission of Sunnah along with other things, what has been transmitted through these means, for it to be classed as Sunnah, it has to be religion by nature, among other things (Principles of Determining the Sunnah: Discussion 56177). In the case of beard, we are not questioning its tawatur, we are questioning its status as religion. Has it been in tawatur as a religious obligation or a non-religious custom? those who establish its religious sanctity do so only on the basis of Akhbar-i-ahad (which unfortunately is also seen out of context).
Thirdly, Ijma and Tawatur are the most reliable modes of transmisison of any historical record/event/information and action. And hence the most reliable source of knowledge. Akhabr-i-ahad on the other hand, are secondary source of knowldge due to inherent limitations in their mode of transmission. Fiqh books incorporate that Ijma, they are not incorporation of akhbar-i-ahad. Akhbar-i-ahad when seen in their entirety, actually contain the application of that Sunnah found in Ijma and Tawatur.
The nature of Sunnah is different as opposed to that of Quran. Sunnah contains actions/practical matters while Quran contains words, and considering their different nature, both are prone to different risks over time. Ijma and Tawatur are used in both these cases to trace them back, as an academic exerise for the scholars. But because of differences in the nature of Quran and Sunnah, the nature of Ijma and Tawatur for both these types are a bit different as well. This doesn’t bring into question the authencity of one over another. The same academic excercise is conducted w.r.t Quran as well (For Example: Discussion 47625).