That is indeed an issue. For now I've just let them stay the early East Slavic culture, but I admit that it really won't do if we're just a tiny bit pedantic (as I tend to be at times). The ideal from my current point of view would be a unique culture for this area (what name it should have, I don't rightly know) even with such a low number of provinces, but that really can not be justified in relation to the amount of effort required. There's also the fact that, even though the naming list (and culture name) is completely wrong as we progress into the 11th century and beyond, the chances that we'll see any, or any significant amount of characters of this culture by then is quite slim.Keep in mind that the East Slavs living in the Kingdom of Hungary (who would later be labelled with the name "Rusyns") became Orthodox the same way their East Slavic neighbors living on the other side of the Carpathian mountains did. If your melting pot is meant to represent Christianized Rus', then those Rus' should also become "russian" culture.
I must agree that, barring the effort of making a unique culture, the best option would be to let them become "Ruskyi" as well. My compromise would then be to significantly increase the time it takes for them to adopt it. I've already slowed it down for provinces bordering West Slavic, Altaic, and Magyar culture group provinces as well as any provinces in the Duchy of Galich. My current setup was to disallow it inside the kingdoms of Poland and Hungary, but with your suggestion I think that will also now just amount to a slowing down.
I think compromise is the central word here.
I have a tendency to let language come to the forefront of "culture" rather than "ethnicity" or "tradition" (even though I'm an Ethnology graduate...) and that is, I think, because what we actually see in the game is not so much "ethnicity" or "tradition" playing out (those tend to have a much higher degree of attention in the game's religions) but instead we see tons and tons of text, and thus language. Now, of course it's silly to use one standardised culture and language for the whole East Slavic region, but with the limitations we have at our disposal, and considering gameplay (and ease) this will do. I simply try to bring some life and development into it, my work (just like my the Scandinavian cultures) being more focused on development (diachronic split up) than division (synchronic split up).Note that there is a reason I keep referring to this group as Rus' or East Slavs (or Old East Slavic when I talk about language), not Russian or Old Russian or Proto-Russian, and I advise you to try to do the same. People who have "russian" culture in CK2 should not be thought of as being tied to Russian language or culture in any direct sense (both of which arguably only really started emerging as their own, distinct entity around the 15th century). They were Rus', they were all Rus', even though Rus'-dom itself definitely was not the monolith that having just one "final" East Slavic culture would imply. The only reason it is even called "russian" in SWMH's game files is to maintain maximum compatibility with vanilla CK2.
It would indeed be interesting to look at making some later melting pots that can trigger after (or alternatively take the place of) the standard Ruskyi culture spreading in some areas like Novgorod. I do think Novgorod (and Pskov) has some attention in later bookmarks already though, with the republic(s) being a nice contrast to the other Russian princedoms. I may have to revise it at some point, since even though I worked on it extensively, it was more patching gaping holes than polishing up a nice shine...If you're going off of the Old Novgorod birch bark letters, one thing you could think about is how this difference in not only language, but also lifestyle (i.e. widespread literacy, a focus on urban culture) and politics (only Novgorod and Pskov adopted republican structures based on their veches, even if veches were present in a lot of other places across Slavdom) came about. One idea could be that this was caused by greater Norse influence in those areas (Norse themselves also having similar, widespread literacy), but then that wouldn't explain why this didn't develop in Kiev. Whatever cultures you choose to represent with your melting pot system, it might not be a bad idea to have something set up to be able to reproduce whatever intra-Rus' split with which you'll want to end up in the 1200's and beyond.