This is my first post here, I'm hoping someone can help me.
I was given a potted hibiscus in 2014 in an non-draining container. I was assured that it was actually better without drainage so it has been in the same non-draining pot for 2 years.. I overwintered it that year indoors in front of a frosted glass door. The room also contained the furnace. Most of the leaves yellowed and dropped. New leaves and flowers came in. I watered it and regularly sprayed it with a soap and water solution.

When I put it back outside in spring 2015, the foliage didn't come back as lush as before and the blooms it produced got smaller. I tried fish fertilizer and that gave a slight boost. Did the same thing this year, adding fresh potting mix and moving it indoors once the temps dipped below 55 F or so. Soap and water, regular watering, the leaves mostly yellowed, dropped, and then regrew over the winter with larger flowers. But many leaves grew slightly malformed, bumpy and curling with one or two unexplained rips in some leaves. Many newer leaves had little "hairs" on them, but the old ones didn't so I figured it was just how new leaves looked. No insects.

This year before bringing it outside I did the fish fertilizer but also pruned it down. It has had some warm (70-80 F) sunny days outside, lots of new leaf growth. Saw some large webs between branches, it was a regular spider. Soap & water misted anyway. Some leaves yellowed & dropped, I chalked it up to changing conditions. Noticed a yellowish dust, chalked it up to pollen. I brought it inside when the temps dipped to 53. It is now back in its overwintering spot.

The yellowing leaves have grayish black veins, as do a few green ones. At least half of the leaves, new & old growth, have dark brown on the serration and tips. I've included a shot of a selection of leaves from the top and another image of the undersides. Is this an iron/ magnesium issue, too much sun, insects, has the lack of drainage finally caused a problem 2 years in, or is it something more dire? So glad I found this forum. Thanks in advance.