spidermites! I think you can see a bunch of them in the last picture in the veins (I think?)
I have a white sapote that I battled with spider mites for like a month and half, the only thing that worked for me was repeated-repeated-repeated-repeated washing. When they are heavy infested they'll make the classic webbing and lay their eggs in the web, but when they aren't established they live mainly on the veins of your plant and will lay eggs inside of the leaf tissue itself.
This is why spraying will not remove them all. The eggs are embedded in the tissue, you MUST wash repeatedly before they mature and are able to reproduce again. The other option that probably works best, but costs a little more, is buying predator mites and letting them go crazy.