One thing to keep in mind is that the time release fertilizer in MG potting soil works by being watered, the "time release" is contingent on waterings, the more you water the more the time release fertilizer's shell/coating is deteriorated/dissolved and eventually breaks and releses the nutrients. I know some other brands of time release fertilizers work differently such as being contingent on temps etc... so correct me if i'm wrong that MG time release doesnt work this way.
As for the discoloration, I would put my money on an Fe/iron deficiancy. And as to what to do about it???? thats a tough one because it could very well be that the MG time release ferts arent working well with the additional nutes youve been watering in? Hopefully it is just a ph or nutrient salt build up that has locked out other nutrients!
Either way at this point all I can think of doing is flushing and making sure your ph is in fact correct (test run off) and maybe get a bottle of micro/secondary nutrients and use at half strength, I can almost garuntee that your problem is not any primary nutrients such as nitrogen. Also I would not use any additional nutrients in that soil unless you start to see a deficiancy of any primary nutrients NPK, with time release ferts you should only have to feed additional nutrients if the plants needs them, watering in extra nutrients is probably only going to interfere with what the time release is made to do.
Moral of the story is: dont use potting soil that has time release fertilizer in it! oh and you should be de-chlorinating your tap water.
Foliar feeding with a micronutrient product a couple of times should speed things up a bit, do not foliar feed directly before or after watering/flushing as your plants will suffocate, and it wont hurt to rinse them with a misting in between foliar feedings.
hope I didnt mess ya all up with all of that, if you have any Q's I'll be around. Goodluck!