Every claim about Eabhadh comes from a manuscript, never from a standing stone. The roughly four hundred surviving ogham inscriptions, cut from about the fourth to the sixth century CE, use only the original twenty letters to record names and, occasionally, short formulas. Eabhadh belongs entirely to the later grammatical tract tradition, chiefly Auraicept na n-Eces and its companion texts, copied by scribes working in a monastic, literate setting rather than carving personal names into a boundary stone.
That difference in medium matters for how confidently the letter can be described. The core twenty letters have a stable stroke system, one to five marks positioned to one side of, across, or directly on a stemline, and their shapes barely vary between one inscription and the next. The forfeda were never subject to that discipline. Their form was set by scribes copying earlier scribes, and Eabhadh's shape, sound value and even its place in the sequence shift somewhat between manuscripts. Treating it as equivalent in authority to Beithe or Luis overstates what the evidence supports; treating it as invented fiction understates a genuine, if late and unstandardized, extension of a working alphabet.
Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.)