It's true that most plants you see pics of were commercially collected in nature. However, things are changing and I've seen plants growing and flowering as well as pics from several longtime cultures. They are a bit slow to propagate but submersed culture seems to be at least as stable as emersed culture.
It does remarkebly well in ... drum roll ... leaf-mold!
Despite it coming from a limestone area, I've not seen it doing well in really hard water (even with added current) like affinis. The first plants I got in 1988 did ok till the mid-90s in pure rain water, natural carbonate-free sand from a lake with some weathered granite added, and really heavy current; too few nutrients for gaining size and propagation though.
I've seen submersed plants doing ok in soft water (with or without current; same with CO2 supplementation; even in small plastic containers); rule of thumb seems to be if a tank allows for really good plant growth without algae covering old leaves, that keei may fine with it. It still is a finicky plant and I strongly recommend to experiment with propagated plants only since the natural stocks are very limited.
I have stable emersed growth in pumice with small amounts of leaf-mold, a carbonate-free mineral mix (sand-like), and pure leaf-mold (preincubated for several weeks in aerated DI water to avoid heavy decomposition (and resulting O2-free conditions) after planting. I suspect that under my conditions the resulting growth is mainly dependant on (low but steady) nutrient availablity rather than more "esoteric" factors.
I hope that some more keei growers will chime in...