I've used the Zygarde Cells to create a 10% Zygarde in Pokémon Moon, am I still able to get a 100% forme, or have I lost my chance?
Yes, you can separate it into cells and cores again or just trade another Zygarde and separate that.
Source https://www.gamerguides.com/pokemon-sun-moon/guide/extra-activities/the-zygarde-cube/building-zygarde#other-functions