Vitamin C and DE are likely not gonna cure schistosamisis. You have a single study that shows this? Probably not where as praziquantel has cured people of it.
Also it's posible d.e. And vitamin c can help but over using a proven drug to treat something doesn't sound like the best idea - plus d.e. likely doesn't go outside the gi tact which dez seems to ignore when pointing out.
Likely longer courses or repeating courses of prazi can help if they are susceptible to it.