Is the key you mean the slab key? because I thought that was used to get the mantle in the swamp.
It shouldn't be possible to get rid of the Green Slab Key until you use it.
Try defeating the final boss again. Make sure to make a new save file when the game prompts. And then load up an older file, and the Green Slab Key should be in your inventory.
If it isn't, try visiting the Jungle Holding Site sub-locale. There's a locked door somewhere in that location that is opened by the key. Maybe you already have visited that place.
so i beat the interloper again and got the key and I think I realized why things got wonky now that I used it I was defeated by him once but got sent back to the tree in the hunter bound suit after being told will broken so I think that was a bug now that I've been to the gallery and used the key. also before beating him again one time my stamina would not regenerate so that is also something I think should be looked into. but now that I've done everything so far I have to say you've done a really good job, thanks for the game!