I once visited a paph nursery in Thailand , which had a white fungal infection in the compost, which they said eventually killed the plants. The only cure they knew was to repot, stripping off all compost, and soaking the plants in a fungicide/disinfectant before repotting in new compost.
I think that the fact that your plant is going downhill is the writing on the wall. Half measures are no good. Be brave !