I don't see how a couple [Hellenic] courtiers in the first two start dates in a barony that the average player wouldn't even bother checking would be out of the question.
If there are Hellenic courtiers, there suddenly are ways to convert to Hellenic without using the console, mods, save game editing, or horse-based exploits (which were unsupported even
before Incitatus was added), e.g. (secretly) converting to your spouse's religion or having a Hellenic character educate your next character (you might be able to prevent some of them, but that's extra work that needs to be done since you'd need to look for all sources of religious conversion and block Hellenic conversion (or use a cleanup event to forcibly reconvert anyone going Hellenic, which is a dirty way of doing it and would cause frustration because there is nothing in the game suggesting that the conversion would be reversed right away)), which some people
would use (and there
is precedent for ensuring that no living non-horse character has an unsupported religion; the Demon Spawn's witch commanders used to be generic Pagan, but when people started to claim that that meant that the religion should be getting some support they were patched to have other religions) and which would become public knowledge very quickly.
The presence of those options makes the devs' position of "We're not supporting the Hellenic faith, and if you somehow convert to the Hellenic faith you are on your own if your save breaks since you've used an exploit to convert" untenable since there suddenly are non-exploity ways of going Hellenic, and they'd be forced to support the Hellenics to at least such an extent that they don't break saves (which is more than they're prepared to do since requests for
that were what eventually made them remove the Hellenics from the Ruler Designer) since to do otherwise would mean leaving a somewhat common (since I suspect quite a few people would try a Hellenic game) source of bugs unpatched.
If they do that, you have a basically featureless religion on your hands (well, a bit less so if you have MNM as the Hermetics are open to Hellenics, but you can't guarantee that everyone has MNM) and also a religion that might have features it shouldn't (because some pagan_group checks might need to exclude the Hellenics due to them being "civilized" pagans operating somewhat differently from most pagans) and lack access to existing stuff it should have (e.g. restoring the Roman Empire, which only Christians can do). This means you'll see requests for features for the Hellenics and tweaks to existing stuff to make it fitting, and the "We aren't adding features to an unsupported (and unplayable) religion (except if it just requires adding another line of code in a handful of places, as was the case for the Hermetics,
and we feel like doing it without being asked to)" argument is no longer something that can be used (since the religion suddenly is supported and playable).
You would also pretty much destroy the main argument against adding other very minor religions and very minor cultures (Crimean Goths, some Roman provinces in Italy early on, etc.), "Culture/religion X was extremely small and largely unimportant in the era, and adding it is too much work", since if the Hellenics are being supported some other equally (or
more) important minor culture/religion is worth supporting too (and this would inevitably lead to requests for e.g. Roman province names because there's precedent for cultural province names and the Romans happened to have names for a lot of stuff). All of the work going into supporting these footnotes is work that could have been spent improving/adding something that had a much larger impact in the CK2 era.
TL;DR: You can't add a few Hellenic courtiers and call it a day.