Ctrl+Shift+F puts default focus on "Find All". So if you press Enter, you will get Find All. If you want to select "Find Next" from this box, press Alt+F while focused on this search window instead of Enter. This tip doesn't change default focus, but it does give you keyboard access to Find Next.
If you want a search box where Enter does "Find Next", try the Ctrl+F search box. In the Ctrl+F window, you could change the search scope from "Current Document" to "Entire Solution". Then the Enter key would Find Next through the entire solution.