Create a design by code in a web application

Yes, exactly. The Web Report Designer can then be used for editing.

Regarding your code, you should call the GenerateLLProject method before opening the view that contains the Web Report Designer (WRD). And then open the WRD for this newly created project. Or - even better, if I understand you correctly - not open the WRD at all but rather have a view that just contains the Web Report Viewer. You can pass the Id of the project to render e. g. in the WebReportViewer tag. Here’s some code: