According to all Bayani sources as well as documentation dispersed between several official non-Baha'i MSS collections (i.e. Browne CUL, British Museum Library, Oxford Bodleian Library, Princeton University MSS collections, Bibliotheque National of France, etc), after a lengthy correspondence of a year where he had elevated him in rank, in April 1850 the Bab officially appointed Mirza Yahya Nuri Subh-i-Azal (d. 1912) as his undisputed successor and mirror. In doing so, and as all the Bayani sources indicate, the Bab widely communicated his nomination to all the Babi leaders of the time and they in turn communicated it to the rank and file of the Babis. One of the original pieces of documentation in the handwriting of the Bab himself appointing Subh-i-Azal still exists and has been published several times in facsimile, beginning by E.G. Browne in his edition and translation of Tarikh-i-Jadid.
In the batch of communications from that April-May 1850 period, the Bab likewise sent a short missive to Mirza Husayn 'Ali Nuri Baha'ullah (d. 1892) instructing him to protect Subh-i-Azal as his sole duty until his death. This missive (whose original in the handwriting of the Bab also exists and is addressed to '238', i.e. Husayn 'Ali) states as follows:
From the element of the Most Splendorous (az ʿunṣur-i-abhāʾ), namely Yaḥyā [i.e. Ṣubḥ-i-Azal], the written tablets (alvāḥ-i-mastūrih), which hath disclosed from the simple substance of ecstatic spiritual attraction, were perused, thanks be the discloser of majesty, majestic and tremendous be he! Exercise a perfect watchfulness over the spirit and vitality (rūḥ va irtivāh) of that considered one [i.e. Ṣubḥ-i-Azal] so that the fierce simoom winds (aryāḥ-i-mushriqa) do not settle upon the heart of that inhabitant; and, in writing and dictation, the means of joy and delight in the acme of perfection should be facilitated for his sake such that he, absolutely, never ever beholds any grief or sorrow, neither secretly nor openly. And take the utmost care of him and his treasured works (āthār-i-makhzūna) until your own time arrives (tā vaqt-i-khud), for God is with the patient! (my trans.)
Nowhere in this communication does it appear that the Bab is speaking to a superior. He is in fact quite emphatically addressing someone who he deems to be a spiritual inferior which he instructs regarding his own successor (who happens to be the addressees younger step-brother).
According to all Bayani sources, as well as those remaining pieces of text from the Baghdad period, for 13-14 years Mirza Husayn Ali Nuri Baha'ullah himself unequivocally upheld (in private and in public) the wording and the spirit of the Bab's nomination and appointment of his younger brother as being unimpeachable. Mullā Muḥammad Jaʿfar Narāqī (d. 1871) confirms:
…in his [earlier] professed declarations of servitude [towards Ṣubḥ-i-Azal], [Bahā’u’llāh] would make the cap of pride and boastful vainglory reach the summit of the heavens…” (my trans.)
According to the explicit text of the Persian Bayan, a divine revelation and its dispensation must reach its stage of maturity until another revelation comes to abrogate it. Moreover, within the Persian Bayan the Bab also states that the Bayan's promised one may appear either in 1511 or 2001 years from his own time. Elsewhere the Bab anticipates a series of mirrors and successors within the Bayan whose number total 19:
…as long as the sun shines all will be illumined by its radiance; and afterwards if one like him should arise then you will be guided by one similar to him, and you will be illumined by the radiance of God until it reaches/concludes with the unity (wāḥid)… (my trans)
In the same text quoted above, the Bab anticipates that such successors or mirrors will arise once every 66 years (which spans a period of 1254 years, i.e. 66 x 19). This clearly underscores that the Bab did not anticipate a future universal manifestation like himself before the completion of this cycle. In other writings, the Bab indicates that the Babi promised one will in the future be born to human parents, will have a childhood and go to school before his or her revelation. The original of this aforementioned document also exist and has been published.
Now, depending on the calculation, Baha'ullah was two, nearly three years older than the Bab (the Bayanis calculate the year of the Bab's birth as 1820 and not 1819 as the Baha'is do). In 1850 Baha'ullah would have been roughly 33 years old and in 1863 he would have been around 46. At the time of his apparent declaration, whether taken in Baghdad or later in Edirne, the dispensation of the Bayan would have been in its most formative stage of a mere 13-17 years old (calculating BE from Naw-Ruz, March 1850 per the Bab's diary). There was no Bayani state in existence at the time per the anticipation of the Bayan. None of the Bayani regulations that required the existence of a Bayani state had been implimented in 1863-7, or since. Most of the remaining writings of the Bab which had not been destroyed by the Qajar state and 'ulama were still in 1863-7 being collected, collated and transcribed in copies. None had been printed yet as the Bab unequivocally antipated in the Persian as well as the Arabic Bayans. Nor had the shrines to the 18 Letters of the Living or to the Bab himself been erected. In short, none of the requirements for the coming of new revelation and dispensation superceding the Bayan existed in 1863-7.
Per the documentary evidence this unequivocally indicates that Baha'ullah had no business declaring himself to be the promised one of the Bayan and to inaugurate a new dispensation superceding it. As such Baha'ullah was not the promised one of the Bayan and hence his assertion regarding the establishment of a covenant has no legal bases and so is legally null and void. When key conditions for the reformulation and/or amendment to an original contract do not exist, or have otherwise not been fulfilled in any form or manner, under any jurisprudence of contract this contract has no legal weight, has no legitimacy and therefore those who attempt to introduce these new stipulations overriding the original contract (in opposition to its stipulations) are in actual fact in violation and so in breach of this original contract. As such this makes Baha'ullah the legally established contract breaker and so Covenant Breaker of the revelation of the Bayan and with him all the Baha'is together as well. QED