Fix for issues with radiator settings on the Toyota GT86 GT4, and Rocket Bunny versions, which under some conditions were causing engine overheating. (gtplanet.net)
Best guess based on your explanation: First the water pump quit / died, then engine overheated as a result and it remained running overly - hot for sometime thereafter, which (likely) led to head gasket failure, which resulted with coolant in the oil and the performance symptoms. (mechanics.stackexchange.com)
You might want to expand your answer to include the possibility of engine overheating: cooling systems are designed to work with water / antifreeze. (mechanics.stackexchange.com)