It is not possible to convert Telerik.Reportign.ReportItemBase-array to Telerik.Reporting.Group-array (or Telerik.Reporting.Group single object) because Groups are not ReportItemBase. The best place to look for examples how to build a report dynamically is the InitializComponent() methods of all sample reports and the API documentation where you can find the Report Object Model hierarchy.
Regarding the width of a section. Sections are logical items that divide the report vertically and as such they are as wide as the report is and you can set only its height. To set the width of a report you should use the Telerik.Reporting.Report.Width property.
the Telerik team