I know it's a year late answer, but here we go.
I think that your tree is probably suffering from Iron and/or Magnesium insufficiency. From what you said you are in area which have a solid amount of rainfall, and that is sometimes a reason for Iron and/or Magnesium insufficiency. On your third picture leaves as it looks has nerve chlorosis, which is sign of lack of magnesium, and on other pictures leaves looks like they are getting more and more pale (sometimes leaves can turn white), which is sign of lack of iron. Chlorosis is abnormal reduction or loss of the normal green coloration of leaves of plants, typically caused by iron deficiency in lime-rich soils, or by disease or lack of light. You can deal with it by adding some micro-macro elements fertilizer by spraying leaves with water+fertilizer solution. Or you can check pH of your ground, and if its higher pH value (above 7), than you should add some acidic fertilizers, like ammonium sulfate, or ammonium nitrate, or SAN, or Urea. If your ground don't have pH value above 7, then probably something else is making your apple's leaves turn yellow.
Of course I can't be 100% sure that this is the reason of your apple's leaves going from green to yellow, but it's my best guess. I hope this helps you. Good luck.
