You have done most of what I would do, except step 3. My eyes are not young enough, so I tend to treat based on probability and symptoms, rather than seeing the critters.
1. treat for insects, especially spider mites, false or true, they are hard to see and more common than most people realize.
2. Fungal disease - best controlled by modifying the environment. More air movement being key. Water in morning to be sure plants are dry before evening. Treat with broad spectrum fungicide
3. There are a couple disease organisms that are neither fungal nor true bacterial. I believe it is some sort of ameboid critter, that lives in the vascular tissues. Your leaves look a lot like the case of seen of this disease. These tend to respond to Copper containing compounds. Phyton 27 is essentially a Copper Sulfate, follow directions, use at the strength they recomend. It is a little pricey, much more expensive than Physan, but cheaper than the more advanced fungicides. Your leaves look a lot like this affliction.
4. If it is only one or two plants, and especially if there are color blotches in leaves or weird blotches in flowers: Assume virus, throw the plants out. There is no cleaning up a virus. Sterilize pots before re-using them.
your problem does not appear to be #5 but I'll toss this in for the other Phal growers.
5. I helped diagnose a problem recently for a windowsill collection, where the Phals would never go much beyond 2 or 3 leaves, even though they were 10 or more years old. Blooming was sparse. Old leaves would develop a similar sickly look then drop off. They seemed weak and susceptable to rots and problems. - Key is that all the Phals were affected to some degree in this collection, not just one plant. Turns out the person was under fertilizing. After checking and doing 1 thru 4, we tried this: The bleaching was malnutrition. They switched to 1/2 teaspoon of MSU fertilizer per gallon of water every two weeks and the collection perked right up. Leaves became green, more turgid and new leaves have develped without old leaves falling off. There is now a flush of flowers coming for the spring Phal season that the owner has never seen before. Especially on a windowsill collection where fertilizing is not convenient, it is possible to starve your plants. The first few years the collection won't show much effect, but eventually the malnutrition will manifest itself, and will be hard to diagnose.
Hope one of these ideas help.
