Maximus got into the Latin by the simple mix-up of two similar words in the Latin.
Nonsense. You are being driven by your predilection for conspiracy theories to make bogus assertions.
All scholars today with only very few exceptions such as Donaldson, i.e. from Zahn ("Der Hirt des Hermas") in 1868 onwards, accept that Maximus is the preferred reading. You may have a friend in Paolo Cecconi who thinks the Old Latin version of Hermas retains importance, but I can't at present find any comment from him on the Maximus issue. However he accepts that different versions of the text must have been in circulation.
Maximus and magna are not similar words. Rather it is likely that someone decided, like you, that they didn't like the name Maximus, as it was otherwise unknown, and changed it (deliberately) to magna. But this change was wrong, given that Grapte and Hermas and Eldad, and Modad are also unknown. Moreover it did not entail a single word modification, as you suggest, but a sequence of word changes, indicating motive as opposed to accident. What seems inconcievable is that anyone would have inserted "Maximus" if the original text had read "magna." A change to magna is the most likely explanation, as per Hoole (below).
The Old Latin "magna" is discordant with most other language versions, including Palatine, Greek and Æthiopic.
Hoole: p171, "The Shepherd of Hermas," 1870,
"Maximus is an unknown person. The name had disappeared
from the Latin versions ..... The name Maximus is
found in the Codex Sinaiticus, in the Codex Lipsiensis, and in
the Æthiopic version " [and in Bodmer Papyri XXXVIII]
Per Carolyn Osiek (Shepherd of Hermas - A Commentary) "Maximus (v. 4), unknown
to us but undoubtedly an infamous object of shame in Hermas' community, is sarcastically single out for a
stinging taunt, and is the shining example of what not to do."
HERMAS LE PASTEUR, INTRODUCTION, TEXTE CRITIQUE, TRADUCTION ET NOTES, Robert Joly, Paris 1958
p.29 "The clergy, however, has its role to play in the announced penance: Hermas the leader, it is he who will make her known, who will exhort to repentance. And ecclesiastical reconciliation? There is no doubt that Hermas attests to this. Maxim[us], to whom he addresses himself in 7, 4, can only be a reinstated lapsus. The apostates of which he often speaks are in the same situation, but Hermas does not mention any rite, any precise ceremony of reconciliation."
"Der Hirt des Hermas," Theodor Zahn, 1868, also accepts Maximus is member of Hermas's household. p.376.