Here’s a quick node map based on a quick landscape I did, showing lots of interconnected biomes, and below that, an example of how a patch can have well over 8 neighbors.
Why should it split in half? I imagine, based on how well and how quickly your new species can compete with your old one, you could end up with a population larger or smaller than theirs, or your population can flourish alongside theirs without decreasing it very much. Splitting it in half kinda feels like an unrealistic and gamey representation of a pretty important game mechanic. I suppose it might work until we have a way to properly determine that sort of stuff, though.
I definitely like the idea of you being the first and only cell to evolve a nucleus. A while ago I did mention the idea of there being a really rare chance for another cell to evolve it as well, so your planet would have two completely separate lineages of eukaryotes (though I feel there’d have to be a significant difference in the nucleus and its effects for that to be any meaningful)