The flower buds that wilt and turn gooey before opening are being affected by disease. It may be botrytis blight, a type of fungus that is widespread during cool, cloudy, wet conditions. As weather conditions improve, it usually goes away on its own. It's also possible that bacterial disease can cause the symptoms you describe, although bacteria usually also create a foul-smelling odor. If you don't notice any bad smell with the gooey buds, it's most likely botrytis. You may help prevent the spread of these diseases by removing and destroying the affected buds.
Are the black spots on your hibiscus on the leaves, the stems, the flowers, or all plant parts? Are they raised bumps or sunken? They could be fungal leaf spots or scale insects depending on these details.