With the Subreport placed in the Group Footer, you cannot show the header on the Main Report's Group Header. You can only show that info in any section below the Group Footer.
To be able to show that info on the Group Header, you need to place the Subreport somewhere above the Group Header section or create two Group Header sections like I said. But what's important is that the Subreport has to execute before the values can be shown on the header.
You can bring back values from the Subreport using a shared variable. So, maybe you can insert a separate Subreport (that points to the same datasource as the other Subreport) and use to show just the Header info always.
-Abhilash