Mealy bugs should be visible either in branches or other parts of the plant. The same applies for aphids and scale. Spider mites require checking both sides of leaves and branches using a magnifying glass since they are as small as periods in this sentence.
I usually have bud drops for one of these reasons: cold temperatures in Spring or Fall; very hot temperatures in the Summer; moving from the nursery to the house; lack of water or over-watering (keep the soil evenly moist during the growing season and allow some dryness during winter); not enough direct sunlight (an indoor problem); pests like hibiscus beetles (appears to have a preference for white, yellow and pink blooms), thrips, aphids, gall midge, nematodes, spider mites, whiteflies, mealy bugs; nutrient problems when root bound in the pot. Note that double-flowering hibiscus is more prone to bud-drop than other variety.
Yellowing of leaves can be caused by many scenarios such as nutrient deficiency, iron chlorosis, pests (spider mites), old foliage, too much heat or watering issues (basically, environmental stress), the use of some chemical agents, root problems, etc. Some yellowing is normal throughout the growing season but a large percentage of leaves yellowing would not be normal. If the leaves develop white spots, the plant is getting too much sunlight.