The plant in question is a Mallika grafted onto Gomera. It is planted in a fabric pot with general purpose potting soil and resides outdoors in a sunny spot while being protected from the rain.
Unfortunately, it's in close proximity to various Citrus trees that tend to attract lots of insects (predominantly mealybugs). For that reason, the plant is regularly inspected for pests (thrips being the most common).
About 3 months ago its health began deteriorating: leaves curving upward/downward and/or inside/outside, brown/black spots on leaves and leaves abscising. Lately, the primary vein of some leaves and some stem tips gradually got black, as can be seen in the following photos.
Black stem-tips
Black leaf-vein
Affected leaves slowly dry up and eventually drop. The blackening of the stem tips seems to progress quite slowly, if at all.
Is there anything that can be done to avert further damage?