It's a fascinating topic, but I don't follow your logic. Or, the Question is off-target? Maybe I don't understand what you seek.
First, antinomian Jews of the Diaspora did not need to repudiate the NEW Moses Myth (after 250 BC) altogether; they could simply adapt whatever then ignore, carry on with their own evolving beliefs. Losers don't write history, though. Second, why must the formula be binary: 'IF NOT Moses, THEN Isaiah..." Instead, I think: what else? In the Diaspora, there were at least several major Judaic cults (so-called Ophite, Enochic, Sethian, Melchizedekian groups) attested to and probably many others unknown/unrecorded (smaller, localized, short-lived, etc.) across the Eastern Mediterranean. Third, although you won't find textual proof as a True-False answer to your question (about alternative OT Prophet cults), a few avenues should be explored further:
1.The oldest levels of Enochic, Melchizedekian & Sethian literature (c.500-150 BC). **I might add Cainite, but I am still uncertain it's an exclusive cult/ literary strand.**
2. Kneph = Yahu at Elephantine 450 BC.
3. Extensive proof of 'Jewish' Serpent worship (confirmed at Israeli dig sites); in heterodox Judaism at Alexandria (archaelogical evidence); by Sinai god associations (Sopdu-Yahweh; Thoth-Moses) which intuitively combine similar Myth & surviving sculpture/inscriptions, etc. (c.1200-450 BC).
4. Myth: Jewish (the Brazen Serpent of Exodus etc.) and Egyptian (The Great Serpent=Agatho Daimon; Hermes Trismegistos - Thoth >>>> Enoch, THEN Moses.) c.300 BC.
5. OT clues.
...
Those are just a few possibilities off the top of my head.
Archaeology STRONGLY supports the Great Serpent worship, in Goshen for example. Moses? Nahhh. There's no evidence I know of which supports/proves ANY Jewish Prophet cult, before Philo's testimony. Besides the Mosaic Brotherhood (probably c.350 BC - 118 AD?), Philo hints at a newer Jeremiah cult (c.25 BC?); I'd love to see more evidence of that, too. But if 'Biblical Moses' is a blatant fiction - say, one Prophet cult whose myth was systematically revamped c.272 BC for Ptolemaic purposes (i.e. to overrule the others) - I'm not waiting on other older 'Prophets' (excl known proof for Seth, Enoch noted above) to be demonstrated as real alternatives in the meantime.
What (group) do you suspect 'Isaiah' is a cover for?