Ms. Matenadaran 2374, a tenth-century Armenian manuscript of the Gospels, has a small marginal note right before the Longer Ending: Արիստոն երիցու - 'By Ariston the Elder'. Could this be the solution to this textual problem?

To begin with, the Longer Ending is absent not only from the earliest Greek codices (ℵ and B), but also from some early Old Latin, Syriac, and Coptic versions. Besides, Eusebius of Caesarea (d. 339) notes that it is absent in "in almost all copies" of GMark.

At the same time, the Longer Ending is present in almost all manuscripts of GMark, as well as in other early Latin, Syriac, Coptic, Armenian and Georgian versions. Besides this, as the quoted tweet observed, Irenaeus of Lyon (ca. 180) is clearly aware of it.

In other words, the Longer Ending was in existence already by the first decades of the second century. This leads us back to Ariston. Papias of Hierapolis refers to Ariston as one of the 'elders' and 'disciples of the Lord' active in his generation (ca. 110 CE).

Similarly, the third- or fourth-century (Ps-)Apostolic Constitutions describes Ariston as the first bishop of Smyrna:

If the tradition noted by the (later) marginal note in Ms. Matenadaran 2374 has a historical basis, this would very well explain Irenaeus, the earliest witness: Smyrna was, very probably, Irenaeus's hometown.

It would also explain why, though the Longer Ending was quite presumably a secondary addition to GMark (cf. Comfort 2008), it is present in most textual traditions: the ending was part of the final redaction / publication of the text by an extremely early, authoritative figure.