I can't say i've ever before felt as tense and hoping that a group of mushrooms manage to board a ship, but here we are.
Nice job on the writing. @Oraculum
Why, thank you. I feared the post might have been tiresome due to its length, but fortunately it appears the lengthy process of writing did produce something palatable after all.
Idea 1 (Related to Oralculum's Races: What about a colony of humans that once tried to settle on Erelvath many years ago in Iurrkhal territory, only to be transformed bit by bit into a sub-species of human (or technically sub-species of Iurrkhal perhaps) due to contact with the fungal Iurrkhal. This transformed these human into a unique species, before the two dominant species of Erelvath drove them off of the planet many moons ago. Since then, the species could have been living in space, perhaps being altered even further into a unique species due to not living on a Garden World for many centuries. That way they could become more unique than just a human-Iurrkhal hybrid of some sort.
I do like the idea - it seems like something which could very well occur naturally, given that Erelvath's biosphere is not entirely friendly to extraneous species and it would have required some adaptation on the part of humans to be able to walk about there without hazardous terrain suits at all (perhaps they could even have done so voluntarily in order to gain an advantage). Additionally, like most fungoids, the Iurrkhal reproduce through spores. Before the Combine introduced specific regulations, these spores were sowed quite openly, scattering over large expanses of ground, and all sorts of strange things could have happened to humans exposed to them for a long time.
On the other hand, we should not forget about Erelvath's other inhabitants, the Vraslil, especially since biological mutation and modification is largely their "hat". The hybridisation process you described could very well have happened naturally; however, I personally find that it could be more interesting if the whole of it had been one large experiment secretly engineered by the Vraslil to see what would happen. Depending on how the humans reacted to their change, eventually discovering this could be either traumatic or, alternatively, give them a reason to be well-disposed towards their "benefactors".
As for being ousted from the planet, that is not something either species would be likely to do, given that they had permitted them to land in the first place. However, it certainly is something the
Covenant would have done while consolidating its power, and the timeline would fit as well.
All in all, I would say this offers plenty of interesting possibilities to work with.