My "appeared overnight" rash was diagnosed as henoch schonlein purpura and confirmed by biopsy. Do you agree there's no prevention, treatment or cure?

No prevention but... The vast majority of henoch-schonlein purpura resolves spontaneously, and recurrence is unusual. Should an abdominal pain develops, steroid can be of help. Renal complication manifested as hematuria requires follow-ups but generally carries a good prognosis.
Yes. The cause of HSP is poorly understood (there are theories, but no clear answer), and there are no direct treatments other than symptomatic relief at this time. The spontaneous recovery time is very short for most sufferers, and any treatment appears to be worse than the disease.
Generally no. While there is no actual treatment for the rash of hsp, there can be side effects in other areas of the body such as the kidney or intestines. Sometimes these other issues that arrise from HSP will need to be treated based on what happens. I would recommend close follow up with your doctor for any worsening or unusual symptoms such as abdominal pain, blood in your urine, joint pain, etc.